If you can monitor all traffic from your router, then you could write an program with the NMAPI to look for Hardware address you wish to block. Then assuming the user has some way to provide a pop-up, you could send that machine a message.
However, if you want to block unauthorized access to your router there are probably better ways. You can limit access by allowing only certain IP addresses in the routers setup. I would look at the router documentation for more details and see if you can accomplish what you want via the routers setup.
Paul