If you want to allow lots of your neighbors to connect and use your wifi and are just interested in monitoring their usage and you have a spare old PC you could try Public IP ZoneCD (www.publicip.net) open source (linux based), boots from a CD. You can setup a complete WiFi zone....give them all free access...but monitor each user, times they use it and how much they download and then go set some limits for each person.
Soo the guy who's taking the p**s with your Broadband gets 52kbps only :D
send him back to the dark days of dial-up...
It'll run on any old crappy PC...with two network cards...pretty straight forward setup too, a few multiple choice options and you're done.
Its been mentioned already but MAC addresses and WEP don't exactly go well with the word SECURITY...and it sounds like you just want to throttle the connection a bit for 1 person...