After logging into the web based interface, go to Advanced --> Network Filter --> MAC Filtering Rules...select "Turn MAC Filtering ON and ALLOW computers listed to access the network". This should prevent any other MAC addresses from being able to access your network. Someone would have to know your password in order to add the MAC address at that point, I'm pretty sure.
You may have to add the MAC address via Setup --> Network Settings --> Add DHCP Reservation, check the Enable box and enter the computer name, IP address and MAC address as a prerequisite, I don't know (that's how I did it, but there may be other ways to do it).