Even still... you can only ban a username and an IP, perhaps an email, etc.
What I am trying to say is that, their is really no true ban as users can come back unregistered from different IPs, or they can just re-register with a different name from a new IP, etc... etc.
I don't know... just doesn't seem to be too practical to ban someone. It is better to leave them around and beat them up. You know the old, keep your friends close and your enemies closer.
Deleting a person's posts every day would have more psychological effect, I would think, than outright banning them anyway.