from Gizmodo  You could just go to a password, or ... Scientists in Grenoble have developed what could be the fix to your neighbors stealing your Wi-Fi. A silver crystal coated wallpaper that can block Wi-FI signals. The crystals are arranged in a way that they block wireless frequencies from your router. 

They are still working on how to block wireless signals that would travel through a window, but give these scientists time. 

It will be released to the public in 2013 and cost about as much at regular wallpaper.