IP banning is a great idea but most ISPs work on the principle that they won't have every customer logging in at once and sell more accounts then they have IP addresses for. Instead they allocate one from a pool when you log in and return it to the pool when you log out. This means that the person you ban today may not be the person usung that IP address tomorrow.
In Europe, where address pools are usually assigned nationally this simply makes banning pointless. In the US where pools are assigned at a far more local level three or four bans on a local site could get your neighbours complaining.
