The wallpaper pictured below is covered in a particular pattern made of silver crystals. The alignment of the crystals means that it is capable of blocking signals used in wireless networks.
The idea behind it is that you cover your walls in the paper and keep the wireless signal you have in your home within the walls and also prevent other signals getting in to the house. Due to the design and layout of the pattern, it does block common Wireless signals but doesn't block regular mobile signals although there is paper that can also do that if required.
Several problems highlighted by Dvice include the issue of windows in your home as well as having to paper the floor and ceiling to get a true 100% block. We assume that covering the walls will somewhat dampen the signals though and provide a little more protection.