instead of deleting accounts or banning people, you trick the bad apples such that they think they can still post and they see their posts with everybody else's posts, but in fact they're the only ones who can see their posts.
Hahaha! That has actually occurred to me. Put them on "ignore" globally and then just let them wonder why no one responds to them.
Of course, the jig's up when either their friends visit the site or they visit the site while not logged in.