So someone is using your wireless without permission, is that what’s going on?
Are you using encryption? A lot of people just assume that any unencrypted wifi network is public on purpose. WEP can be cracked by any half-assed script kiddy in a few minutes, but if this person can crack WPA2 you might not want to mess with them…
Probably you already know this, but most routers have the ability to filter by MAC address built-in. Unfortunately the MAC address stays the same when the computer reboots, so that’s not quite what you wanted. You could probably boot them off by releasing their DHCP lease if the router supports it, but that would only work until they or their network manager software ran dhclient again. Does your router run the stock firmware, or something like OpenWRT (or is it the router running BackTrack)? That pretty radically affects the options available.
Also, it’s pretty easy to spoof a MAC address (hell, even I can do it) so that might only boot them temporarily anyway.