I would fit a router as suggested above.
Now if you register the router as the primary MAC address associated with your property, rather than clone the PCs address, if he pulled the cable out of the router and connected it back to the PC then he still would not have access.
You can also provision the router to not allow specific site access with certain rules, even when he has permission to surf.
ftp://ftp.linksys.com/pdf/wrt54gv1.1_ug.pdf
The linksys instruction manual showing the various option on that product. Most of the competition are similar.