As much as I hate legal stuff, a harassment or slander charge could be in order for some of the real idiots.
Some of these people simply continue to come back, canceling accounts won't do it for them. They simply have no life. When I worked for an ISP, it was amazing how effective simply beginning a legal action was. Not threats, but action. They shut up quick after a law enforcement person or legal courier delivering papers stops by.
It doesn't have to be expensive, either. All it has to cost is a small chunk of lawyer time and the costs of filing and serving the papers. Not $20, but possibly worth it for the real idiots.
Barry