What's a word for a person who denies all knowledge?

Within the UK police force this is (sometimes) known as 'the Shaggy defence', in reference to the song "Wasn't Me".

I can't find an online source for that. I only know it because a police officer friend of mine told me about it.

Oh, actually, here we go:

The Shaggy defense

"I predict that in the decades to come, law schools will teach this as the 'Shaggy defense'. You allege that I was caught on camera, butt naked, banging on the log cabin floor? It wasn't me."

Slate

PESCA: Good. R. Kelly's defense is really pretty simple. He's just saying it wasn't me, and the defense team also says the girl in the video, or the girl the prosecutors are pointing to, it wasn't her. You saw the tape. The jury saw the tape. How does that defense work? What - does the guy look like him?

Mr. LEVIN: The guy definitely looks like him. As he said, it - the it-wasn't-me defense, which I'm calling the Shaggy defense, after the popular reggae song.

http://www.npr.org/templates/story/story.php?storyId=90755482

Meet the Shaggy Defense: Lying in the face of overwhelming evidence.

http://www.aolradioblog.com/2010/09/11/100-worst-songs-ever-part-three-of-five/

The defence favored by the protagonist in Shaggy's song "It Wasn't Me". Despite overwhelming evidence to the contrary, he maintains his innoncence by simply saying it was somebody else. Also liked by R. Kelly, O.J. Simpson, and Phil Specter

http://www.urbandictionary.com/define.php?term=The%20Shaggy%20Defence

It’s now official: The “Shaggy defense” is a term of art

When Danville U.S. District Judge Jackson Kiser rules that the “Shaggy defense” won’t fly in a summary judgment motion, we can safely assume the term is now ensconced in the annals of legal terminology.

For those whose musical tastes don’t include R&B, the “Shaggy defense” refers to a 2000 hit by reggae star Shaggy (Jamaica native and U.S. Marine) who counseled a wayward lover to deny everything with the phrase “It wasn’t me,” despite all evidence to the contrary. The term is helpfully explained in footnote 3 in Kiser’s opinion in the case of Preston v. Morton. Kiser notes the term “Shaggy defense” gained popularity when used to describe another musician’s defense in a criminal case.

In the civil case before Kiser, Preston claimed he was working on traffic signals in a bucket truck and was injured when his bucket was hit by a passing tractor trailer driven by Morton. Morton’s main defense was, you guessed it – “It wasn’t me.” Kiser said the issue of whether Morton was the driver in question was a disputed material fact, holding “the so-called ‘Shaggy defense’ is inappropriate for summary judgment.”

http://valawyersweekly.com/2010/05/25/its-now-official-the-shaggy-defense-is-a-term-of-art/

since the person in question is putting up that act i suggest that person should be called 'knowledge refuter'.

In England, such a person might be termed "a Manuel" after the character in Fawlty Towers, whose most popular catchphrase (delivered with brio in a cod-Spanish accent) was, "I know nothing."

In philosophy, the school of thought which denied the possibility of knowledge is the Skeptics, so the word would be Skeptic or Skeptical.

From Wikipedia:

It is generally agreed that knowledge requires justification. It is not enough to have a true belief: one must also have good reasons for that belief. Skeptics claim that it is not possible to have an adequate justification.

Skepticism is not a single position but covers a range of different positions. In the ancient world there were two main skeptical traditions. Academic skepticism took the dogmatic position that knowledge was not possible; Pyrrhonian skeptics refused to take a dogmatic position on any issue—including skepticism. Radical skepticism ends in the paradoxical claim that one cannot know anything—including that one cannot know anything.

In medicine, the phrase "in denial" is used to describe a psychiatric condition.

From The Free Dictionary:

denial [dĕ-ni´al] in psychiatry, a defense mechanism in which the existence of unpleasant internal or external realities is denied and kept out of conscious awareness. By keeping the stressors out of consciousness, they are prevented from causing anxiety.

ineffective denial a nursing diagnosis accepted by the North American Nursing Diagnosis Association, defined as denial that is detrimental to health when a person makes a conscious or unconscious attempt to disavow the meaning or even the knowledge of an event in order to reduce anxiety or fear.

Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

In Law, a general denial rejects all the material elements of a petition, whereas a specific denial rejects particular elements.

From The Free Dictionary:

Deny To refuse to acknowledge something; to disclaim connection with or responsibility for an action or statement. To deny someone of a legal right is to deprive him or her of that right.

A denial is a part of a legal Pleading that refutes the facts set forth by the opposing side. A general denial takes exception to all the material elements of the complaint or petition, and a specific denial addresses a particular allegation in issue.

West's Encyclopedia of American Law, edition 2. Copyright 2008 The Gale Group, Inc. All rights reserved.