The way our team would deal with it would be to look at the IP he was using when you banned him, and/or the IP of his forum account, and ban if they match or are in the same city (do a Google search for "IP locator " and find one you like, I use GeoBytes), then you can feel confident that it is the same person, especially if it is a specific color/caps match or is a very unique name (I probably wouldn't ban if it were a name like "Killer" or "SniperGod"). I'm pretty sure this is standard practice for zero-tolerance clans that do good research.
Does it look like he's hacking now?