Hmph jawert you beat me to it
If you want to get super technical....
Allow only certain MAC addresses on teh router (MAC addresses are TOTALLY unique to each network card...no two alike kinda deal) all you have to do is find the MAC addy of your PCs and allow only access to them
Find the MAC on your pcs:
1. Start -> Run
2. Type in "cmd" and hit enter/OK
3. At the little black window type in "ipconfig /all" and hit Enter
4. Find the MAC address of the network adapter (ie. if your PC is connecting wirelessly use the "wireless adapter"...if its on a line use "local area connection")
5. Write it down
6. type in "exit" or hit the red x!
7. Set up your router to allow only certain mac addresses
8. save changes and watch him try! lol
That'll block 'em out! lol
If you need more help lemme know