It's not illegal to have copper wallpaper or whatever, if you want to go full faraday cage. Passive signal blocking is legal. There might be a few situations (movie theatre?) where you would want to install a big obvious sign stating that wireless devices will not communicate outside the room. It's really no different than being in the third level sub-basement of a concrete parking garage.

Jammers are not legal, unless you're operating them in a shielded RF test chamber.

It’s illegal to jam signals but I can’t imagine it’s illegal to shield from them. Just concrete does it well enough.