It is tempting just to pee off the neighbours, but what is wrong with just putting a password on your wi-fi for starters?
Had to be invented in France didn't it?
http://gizmodo.com/5909100/wi+fi+blocking-wallpaper-protects-your-web-fortress-by-keeping-neighbors-out (http://gizmodo.com/5909100/wi+fi+blocking-wallpaper-protects-your-web-fortress-by-keeping-neighbors-out)