Bann dosent do anything

Well this guy i want banned off my forum so i bann his account name and his ip from the site. But it dont do anything cause i think he runs a proxy server and then makes a new account and gets right back on.

How do i get him off the site?

Not much you can really do with that. First thing dont ban them anymore.
the best thing would be to not allow any new members permissions to the forums, then have a welcome/introduction board where all new members must post and say hi to gain accesses to the rest of the forum.
Then once they post you can see if the ip matches up with any members you do not wish to allow and will know not to grant them access.

Like i said above do not ban them anymore.. Just place members you would ban in a hidden group that has no permissions to do anything on the forum.. He will just be stuck with no access or permissions.. and will make it easier to check ip to see if they match with any members that you may not want there.
(meaning it would be the same person) when you do not ban ip or members when you check a ip it will also show other ip and or members that may have used it.