Good one, it depends on how much money you want to spend, you can get good access points that will sort of DOS the other signals, but then you aren't being a good neighbour
You can find them easy by the signal even something like WiFiFoFum on a PDA will allow you to track to the location. My neighbours have a wide open linksys as well, but it's very useful for testing my perimeter
Like others have said, you can lock down the PC's with policies as long as your kids haven't already installed keystroke loggers.
Good Luck