Tom Chick, owner of QuarterToThree.com just finished a controlled clean up of his forums where several prominent users were shown the door.
Moderation is a very delicate and thankless art. A community can easily be taken over by cliques of griefers without careful moderation.
I think JoeUser.com users can remember a certain user by the acronym of LW who was a very prolific and interesting poster but yet couldn’t keep herself from griefing those she disliked.
Over on Neowin.net, we’ve struggled with this issue for years on how to have a community that is interesting and fun but can keep the bi-polar crowd from getting out of control (as someone who tends to be a bit bi-polar, I understand the issue but you have to just shut the heck up when you’re in one of the extreme moods).