The Verge:

French researchers have developed wallpaper that's designed to trap Wi-Fi signals, without interfering with radio or cellphone signals. It uses conductive ink containing silver crystals to to block a Wi-Fi router's operating frequencies: your router should work as expected, but the signal won't travel beyond the wallpaper's boundaries. While currently only a prototype, researchers at the Grenoble Institute of Technology hope to make the wallpaper commercially available early next year.
I think I'm the last person in this city who doesn't lock his/her Wi-Fi. It is open. It will remain open. I will never surrender.