hi mate i guess you could give the xbox mac address a static ip address so every time that it's turned on the router will give it the same ip address.
Then you can block the ip address in the router.
You could also go into xbox live settings and give it a static ip address, some thing stupid like 192.168.168.168 and then disable the dhcp settings, this means that every time the 360 was turned on it would be looking for an ip address that doesn't exist.
to give static ip addy on the router.
turn everything on and have a look in your router page for lan ip set up, if you go to add then you should see the 360 mac's addy, add this so now every time you start the 360 it's given the same ip. (it should add the mac address and the service automatically when you click add"
192.168.1.5 xbox 360 00:17:FA:53:12:8C
Then you can block it under " block services"
might as well do the 360 though, it be easier...