Banning someone on a proxy is pretty much impossible. You can ban by username and ban by IP. If their IP is always changing then that's about it. You could require approval of new memberships before they are active, and maybe you could spot him.
There are some mods at threadsdev.com to help detect duplicate users by cookies, (Multiple Identity Detector) but your version is so old, not sure if any of the old instructions kicking around would help.
