Good point.
There is simply no way to ban any person based on the IP they used. You can only ban a person based on IP if you remove the ban after X hours.
If you can't ban people, you can at least make it less interesting to keep signing up
People are less likely to keep signing on after a ban if the signin process requires a new emaill address, replying to a confirmation email etc.
Another simple thing you can do is set a cookie. If a banned person tries to sign up, let him go through the process, and as a final step, check for the cookie. If the 'you are banned' cookie is present, send the user a 'nice try!' message. Sure, it's a piece of cake to unset the cookie, but if you let him go through the signup, he has already entered his email, and so he now has to unset the cookie, get a new IP, and get a new Email before he can try again.
