Hi.
Ok, I feel using a mac-filter address or WEP encryption key isn't really the solution for you. It's quite easy to crack your key and to change the mac-address to one of your neighbors..
If you want to find who he is, you could try simples tools like :
- wireshark : You can snif packets of the guy and maybe, you'll find pieces of information about his identity.
- nbtscan : it will try to gather netbios informations of the computer and who knows, yu could find a name :)