Can the word "ignorant" be used as a noun?
In theory any adjective can be employed as a noun—in fact, until the middle of the 18th century western grammatical theory put adjectives in the same category as nouns, distinguishing only between 'adjective' and 'substantive' uses of the noun. And in many modern European languages adjectives are employed as nouns very freely: in German, for instance, the only difference between an adjective and a noun is that the noun is capitalized—der arme Mann and der Arme both represent E 'the poor man'. French (le pauvre) doesn't even make that distinction.
However, English does differ from other languages in restricting nominal use to adjectives which represent readily recognized categories. The poor, the rich, the ignorant are acceptable collective nouns, designating all poor/rich/ignorant people, but we do not ordinarily use the rich to designate a specific rich person. That sort of use occurs only in discourses where the adjective has previously been established as categorial. For instance:
Do you want one of the red t-shirts or a black one?
I'll have a red.
CGEL treats this sort of use as an instance of the adjective fusing with its implied head, just as determinatives like which and some fuse with their implied heads to constitute pronouns.
Ignorant is an adjective, Etymonline cites its usage as a noun also. I think it is an archaic usage. Note that the Spanish (ignorante) and the Italian (ignorante) are both adjectives and nouns, so the Google results might be from inaccurate translations:
late 14c., "lacking wisdom or knowledge; unaware," from Old French ignorant (14c.), from Latin ignorantem (nominative ignorans) "not knowing, ignorant," present participle of ignorare "not to know, to be unacquainted; mistake, misunderstand; take no notice of, pay no attention to," from assimilated form of in- "not, opposite of" (see in- (1)) + Old Latin gnarus "aware, acquainted with" .
Form influenced by related Latin ignotus "unknown, strange, unrecognized, unfamiliar." Colloquial sense of "ill-mannered, uncouth, knowing nothing of good manners" attested by 1886. As a noun, "ignorant person," from mid-15c.
To get a sense of historical use of ignorant as a noun, I ran a Google Books search for the plural form ignorants—and found a fair number of matches, stretching back to the 1600s. Here are some examples from various points along the timeline.
From John Seldon, The Historie of Tithes: That Is, the Practice of Payment of Them, the Positiue Laws Made for Them, the Opinions Toching the Right of Them (1618):
Therefore I suppose (if he haue not studied the Laws, or otherwise know it) he will rather take some minuts pains then blame me for not turning it. and howsoeuer to diuers peeuish Ignorants, out of their daintie stomachs, and a pretence of nothing but the more polished literature, it may here seem barbarous and distastefull; ... If to so great a man [Father Gregorie] that curious language could seeme no pleasanter when he studied it, it is the lesse wonder that the Law French (which doth as truly and fully deliuer the matter in our Lawes, as the Latin in the Imperialls ; though indeed farre from polite expression) should bee so contemptible among the many petie Ignorants which vsually despise what euer their lazie course of studies hath not furnisht them withall, ...
From Edward, Lord Bishop of Cork & Rosse, A Practical and Plain Discourse of the Form of Godliness, Visible in the Present Age, and of the Power of Godliness (1683):
Now there is no doubt but both and all thefe, our Church ignorants, and our Outlying-ignorants, and that whether through supine negligence and sloth, or through resolute design and obstinacy, all of them, I say, Deny the Power of Godlineß, whatsoever they may have or retain of the Form : For they stop the very first passage by which Godlineß can come to their hearts, or ever have any power over them ; namely they suffer it not to enter into their Understandings.
From Abraham Cowley, "Bathing in the River," in The Mistress: Or, Several Copies of Love-Verses (1720):
Why to Mute Fish should'st thou thy self discover,/ And not to me thy no less silent Lover?/ As some from Men their buried Gold commit/ To Ghosts that have no use of it!/ Half their rich Treasures so/ Maids bury; and for ought we know/ (Poor Ignorants) they're Mermaids all below.
From "Debate in the Lords on the State of the Civil List" (March 14, 1770), in The Universal Magazine of Knowledge and Pleasure (1770) [combined snippets]:
The Earl of Chatham spoke in support of the motion. ... In every other respect, the minute and particular expences of the Civil List are as open to Parliamentary examination and inquiry, in regard to the application and abuse, as any other grant of the people to any other purpose; and Ministers are equally or more culpable for incurring any unprovided expence, and for running in arrear this service, as for any other. The preambles of all the Civil-list acts prove this, and none but children, novices, or ignorants, will ever act without proper regard to it; and, therefore, I can never consent to increase fraudulently the Civil establishment under pretence of making up deficiencies; ...
From J. Ewing Ritchie, "The French Revolution of February," in The Metropolitan Magazine (April 1848):
And so wretched was the instruction given [in France], that we find a large proportion of the criminals amongst the educated, and not the uneducated classes. In the most ignorant of the French departments, Correze, with eight hundred and four ignorants in one thousand young men of twenty-one years of age, there was but one criminal in ten thousand three hundred and sixty inhabitants. In the department of Creuse, where the ignorants were six hundred and twenty-one to one thousand, there was but one criminal in twenty-one thousand inhabitants; whilst in the Seine Inferieure, with three hundred and forty-six ignorants, and Seine et Marne with two hundred and twenty-eight, the proportion was four thousand four hundred and ninety-six inhabitants for one criminal, in the first, and in the second, six thousand one hundred and ninety-four. The general average of departmental criminal statistics gave one convict for every six thousand three hundred and seventy-nine inhabitants, and four hundred and eighty-four ignorants in one thousand men; while in the forty-three least criminal departments, the ignorants ere five hundred and eighteen in one thousand, and one convict in eleven thousand eight hundred and twenty-nine inhabitants.
From Seymour Supercern, Truth Will Out: An Emergence from Chaos and Subterfuge Into Light and Reality (1912):
Hypocrisy! mean, sneaking weasel, means self-deteriment,/ But Bigotry means self-annihilation, as 't has meant,/ In times gone by when nations slaughtered, slew, like fiends run loose,/ Harangued and tortured, murdered, butchered at the slightest ruse,/ As 't has meant when ignorants, those would-be pious pests,/ Fall down with faces in the bogs, callous to all behests,/ To rise them up from foul and festering filth, where all is night,/ Cimmerian as a Stygian realm; ah, pity 'pon the bigot's plight!/ As it now means, as it has meant, still grovelling in the bog,/ Till grown to it, like marshy mosses to a swamp-lain log./ Though mitigated nowaday if not eradicate,/ It still holds sway 'mongst unenlightened and 'twill not abate,/ Until the ignorants will be enlightened in such wise,/ As to make them see through things alone, a self-wrought willful rise.
From American Scientist (1942) [snippet view]:
Here is how they begin:
The stochastic theory of rumours, with interacting subpopulations of ignorants, spreaders and stiflers, began with the seminal paper of Daley and Kendall.
...
Spreaders communicate the rumor to ignorants, turning them into spreaders; when two spreaders meet, both become stiflers; a spreader who meets a stifler also becomes a stifler. The process begins with a single spreader in a sea of ignorants.
From Intrapreneurial Excellence (1985) [combined snippets]:
The Innovation Ignorants. Made up of some of America's best known companies, this group of firms still does not appreciate the close correlation between longer-term growth in return on equity, sales, and earnings and the skills required in the management of innovation. Typically nothing will change the behavior of the innovation ignorants until one of two things happens. Either they get into serious trouble in their markets and are forced to say, "We've got to learn to manage change," or they experience a change in leadership that leads to a new set of organizational values.
From Arnulf Depperman, "Using the Other for Oneself: Conversational Practices of Representing Out-Group Members Among Adolescents," in Selves and Identities in Narrative and Discourse (2007):
Just before the transcript starts, one part of the youngsters (the "savants") played a trick on the others ("the ignorants") by requesting them to hand them objects (a bottle, a toy cow, etc.) that the savants had glued to the walls and to the desk of the caravan. The ignorants' failure to lift the objects pleased the savants, while the ignorants reacted with irritation (see Mark's insult in line 01).
Conclusions
From these results it appears that ignorant[s] was in general use in the 1600s and on through the early decades of the 1800s, with the meaning "ignorant person or people." Noah Webster, A Compendious Dictionary of the English Language (1806) offers this entry for the word as a noun referring to a type of human being:
Ignorant, n. one who is untaught or unskilled.
In more-recent decades, the term ignorant[s] has popped up in the fields of psychology, business, and social science—in each case with a fairly narrow and specialized meaning.
The upshot of all this is that, although a great many native English speakers would view calling someone "an ignorant" as nonstandard English, the word used as a noun and applied to people has a past and a present in the language—and may very well have a future, too.