I can answer both interpretations of the question.

If you mean in fanics, then if they are actual replacements, we kill them just like normal sues. If they're just dramatically out of character, we restore them either by killing the Sue responsible, or by beating them over the head with a canon text until the author-wraith is exorcised.

If you mean characters in actual published stories, then there's not much we can do but complain - the Eragon books are a notable example. Strictly speaking, as PPCers, we're supposed to defend them, too, but if a character isn't much liked, most agents can easily find pleanty of more worthy souls to watch out for.