Assuming you want to leave the internet connection available, uninstall the programs and periodically check to see that they’re not re-installed. Also, keep an eye on him.
In all honesty and directness? Remove the network cable/network card if computer use is still needed to do homework. If homework requires Internet access, too bad. He can use the computer at school.
Think of this as an addiction on his part. Think your response to his addiction as intervention/tough love.
If you’ve got Windows Vista you can create an Administrative account and set up Parent Control, in which you dictate when and for how long he can use the internet and which programs are and aren’t restricted. (I don’t know if you can do this on Windows XP or any other operating system).
I doubt Diablo has any meaningful DRM, so he could just borrow friends disks and re-install. But if its a more modern OS, you could manage his permissions to keep him from being able to reinstall anything instead of just taking the disks.
Pull his graphics card and use the onboard if available or the shittiest card you can find. I’m assuming he doesn’t have a couple hundred to drop on a better card.
Tht’s how we went from WOW and and Nvidia 8800 to Diablo using onboard video. He just went back to the old school Diablo that would play on his machine (and was free) when I pulled the video card.
Diablo II requires the game CD to be present to start. Take away the CD, poof, “No Diablo for you!” even if the game is installed. Note that there are two versions of the game, the base version and the expansion version. He’s probably playing the expansion, which means the game CD would be “Diablo II: Lord of Destruction”. This won’t help if he has friends willing to loan him their CD, though, but that would mean that they couldn’t play the game, then.
But for any technological solution there is a technological workaround. And with a sufficiently motivated user, a workaround will eventually be found. The best long-term solutions are social solutions, such as to placing the computer in a public area that is easily viewable from different parts of the house.
As for World of Warcraft, you could get in contact with his guild, and ask them for help. If there are responsible folks in the guild, they could provide additional social pressure on him, too.