Site admin responsibilities

My ethical dilemma for today. I know, because I’m the site admin, that a particular user is posting things under multiple names, agreeing with himself to lend credibility to a position that nobody else holds.

So, the question is, when users have an expectation of anonymity, and one user is intentionally exploiting that anonymity to be cruel, can I “out” this user, because it’s my site?

I dunno, but he’s seriously making me angry, and I suppose I should just take the moral high ground and ignore him, but I’m finding that very hard to do.