At some point it probably makes sense to line each apt in rf blocking sheets and then have only your own wifi inside. Would cause problems with guests on 4g though.
It would also drain the battery of any phone you forget to switch to wifi-only. Most phones, when dealing with tenuous or absent xG signal, will try harder and harder to transmit.
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.