A new paint mixed with nanoparticles may offer the sweet silence that theatre owners, school officials and clergy crave. Once applied to walls, the paint blocks cell phone radio signals, locking out any call. NaturalNano, a firm in Rochester (New-York), commercializes the materials.

Two-way radio-frequency transmissions from cell phones, WiFi, and other electronic devices may be blocked.

Nanotubes coated or loaded with copper may be used in paint applied to walls to passively block specific radio frequency ranges. That makes the difference with current techniques which scramble ALL received signals.
Indeed, the wireless phone industry is opposed to blocking technologies, citing legalities and various emergency calls: police, fire, ER, ...
NaturalNano has the solution: Using a complementary technology licensed from AMBIT Corporation, building managers may selectively override the blocking to allow the use of two-way devices in a room or building. An indoor antenna picks up cell-phone or other signals and sends them to an external antenna through an electronic filter

Source: the newspaper Communications of ACM.
For those who do not know: nanoparticles are a direct application of nanotechnologies. The small tubes on the picture here above are 40 nanometers length, or if you prefer 40 x 10^-9 meter, or again 0,000000040 meter. Or 400 atoms. This microscopic size makes it possible to block waves because they're lost after tons of rebounds.