Maj is asking for a method of banning *specific* aol users, mainly because the way aol is set up would make an ip ban impossible for a single user.
My suggestion is to contact aol and give them the user's address and list the infractions you've had to issue. It's a pain, but it may be your best alternative.
Actually, I've installed this (Miserable Users mod), and I have to agree that it's an excellent way to deter a problem user, and is probably 10 times more effective than banning.