I can't believe all you get in the comments are psychobabble. It's good up to a point but if you're past that, the easiest way to save him from himself is to fuck the internet connection.
No internet, no game.
Get BackTrack 3, boot into a spare PC and try TCPKILL-ing his connection to the specific port. You can script it to kill all connections to port (blah) every 5/10/15/rand() minutes.
If he has a clue about networks, he could realise what you are doing, though, so even better would be:
Limiting the speed at which he can connect, start by setting a 50KB/s limit, then 25KB/s, then 12, then 5, 1, whatever.
He will not be enjoying his game if he is getting 2 FPS and is completely lagged and out of sync from the virtual world.
A way that comes to mind is Man-in-the-middle attack (focused on the port(s) used by the game) and after all game traffic is going through you, use one of Trickle, Level 7 Filter, ClarkConnect, Bandwidth Arbitrator, MasterShaper.
If you can't figure out how to throttle him properly, you can also try flooding the local network until it slows to a crawl.
Try it.