It would be obvious if you've blocked it.
Assuming you have control over the router, log into the admin page, change the admin password so he can't log in himself. Then change the wireless password key. You'd have to change the key on your own computers to match the new one.
If he has physical access to the modem / router, he could of course just unplug cables.
Edit: Oh and do the SSID thingy as suggested above too