What is the most appropriate noun for 'a person who is ostracized'?
It’s unrelated to ostracise, but how about outcast?
Ostracism comes from Greek ostrakon (related to osteon, “bone”), which referred to the clay ballots on which individuals would write the names of those they wished to banish from their community. Outcast is much more literal, and a much older word in English.
A person who is ostracized is a pariah:
pariah
noun
1 an outcast: they were treated as social pariahs.
[source: NOAD]
A third, somewhat legalistic possibility is persona non grata.