They not been paying for the internet then?
Two simple ways, off the top of my head:

If your housemates aren't computer savvy, set it to fixed IP instead of DHCP (automatic asign). Then you put your IP address in manually and you can connect, their computers will look for a DHCP server and not find one so won't be able to connect unless they think of setting their IP manually, and then they'll need to know what range it's in.

Or just change the wireless network key (I assume you're using an encrypted connection?) and don't tell them the new one! This assumes everyone is connecting wirelessly though, it won't stop anyone plugged into the Cat5 ports.

There's more advanced shiznit you can do like filtering on MAC addresses and only allowing certain IPs to connect and things, but I'd have to know which router you had - a lot of your basic home ones don't have this sort of functionality.
(Son of Sam-i-am We're all as mad as hatters here, Tue 28 Oct 2008, 11:23, Reply)