I would first be sure that abusive behavior is clearly defined in the TOS such as profanity or name calling won't be tolerated, etc. If you state the behavior and any consequences unequivocally, you have grounds for "firing" the customer. Unfortunately, many people have had "the customer is always right" drilled into their heads and some think they can get away with any sort of behavior. You shouldn't have to put up with bad behavior online or offline.