Does your router have MAC address filtering? If so, turn it on to accept just the allowed computers. That's what I did, after having many problems with a mysterious client. I live in a relatively built-up area, with quite a few wireless networks; I added WPA encryption, and changed it to a really long and nasty password, and that seemed to work for a while, but it just didn't cut it, apparently. There's nothing more unsettling than someone stealing a share of your bandwidth.
"This is the worst day of my life."
"Correction, the worst day of your life so far"
- Homer Simpson