Ton there's another way, but your roomie isn't going to like it. First you have to be the primary computer on the network, and it sounds like you are.
If you can't throttle the bandwidth on his NIC, you're going to have to play a little dirty pool. Sounds like you might not mind that anyways, as he is sucking your bandwidth dry and may not care.
You are the server, and since you haven't mentioned an external router, you are the router and the DHCP server also. Simply exclude his IP address from the DHCP scope or block his IP address from accessing the router whenever you need to shut down the Napster stuff. Or change his hardware profile to have the NIC enabled and put the profile selection time on bootup to zero seconds. Change his user.dat file to user.man and he won't be able to make any changes to his profile.
Get him a modem for his computer and set him up with a free (really) internet account with
http://www.seniorsinfree.com Use a proxy server like Winproxy and disable the ports that he uses to access Napster or any high bandwidth ports like FTP.
Put a 10 meg pos NIC in his computer when he's out and set it to half/duplex. Change his protocols so you disable TCP/IP and only enable NetBios, so he can communicate with the workgroup, but can't access the outside world because that protocol is not routable.
That should give you a few ideas to work with I guess. 8)