Well here's an interesting niche market. A company called Force Field Wireless is selling anti-WLAN paint!
"This specially formulated flat interior paint will help reduce the transmission of radio waves through walls, ceilings and doors. A great solution for protecting your Wi-Fi, Wi-Max, Bluetooth or any wireless network in the 5GHz or less frequency range. Stop radio interferences from slowing down your network. Prevent your wireless data from being hi-jacked."
