Cell blockers are illegal in the US. Even when the power is very low, if there's enough circumstantial evidence to prove intent to interfere with a transmission, I think that's an $11,000 fine from the FCC.
Perfectly legal to install EM shielding or a Faraday cage. I bet you'd only have to install a few of them before the mobile companies agreed to block certain urls whenever they triangulated a phone as inside one of your stores.
I wonder how casinos get away with seemingly blocking cellphone signals. In my experience whenever you go near a slot machine or table game in a casino, cell signal drops to zero.
