Of course if you do determine that it's those "punks next door" ruining your wifi coverage by occupying the only available channels, you can always do what I did: get an external 2.4 Ghz antenna and a 2.4 Ghz amplifier... and just obliterate the channel they're on with your own signal. Sure, it won't be terribly practical or useful, but at least it should cause enough grief for them to up and change their channel/etc. If you're feeling really annoyed, get an older 2.4Ghz cordless phone (don't connect it to the phone line or anything), and just leave if off hook for a few days... again being sure to place the base station as close to your "next door" as possible ;o)
(the "neighbour friendly" approach of course is to do what everyone's suggested and change channels... I'm just offering some... alternatives)