This could also possibly encourage more bad behavior. After all, if the site is going to treat them like a bad actor with no hope of redemption, why even try to avoid trolling?
Why not do something quite simple. Have it setup so that if a user has over a certain karma threshold they can flag a user's post in some manner as being mean-spirited or other such. If a certain number of users also flag the comment, then the poster does not get any of the karma points their comment may have earned. Alternately, the commenting flagging could be coupled with a slashdot style moderation and meta-moderation system.