He is probably on DHCP.
It is not like his behavior is not documented, "you cant stop me admins!".
Send a cease and desist to his ISP. This is harrassment plain and simple.
Or, if you can block e-mails it could just make it a little harder. IPs are much easier to come by than emails although emails are pretty easy to come by now adays too. Argh. I don't know. Just let the little bugger struggle to come back with the loophole called the internet, wasting his time, and providing a momentary chuckle as he gets banned again and again. If he is really that persistent then my original idea will have a lot of weight with his ISP.
-Nasser