Actively blocking cell access is illegal, as in you cannot set up RF jammers or other such equipment. It is a violation of FCC rules. This is because you cannot control the dispersal pattern of the offending signal. Now passive blocking is perfectly legal, but rather expensive. There is
RF blocking paint (and it has uses outside of the cranky, "the EMF is making me sick" crowd) as well as the ability to shield RF with metal screens (Faraday cage). Think of your microwave and that glass window, there's a Faraday cage there to protect you when you look inside to see the milk boiling over. Our Co-op which was built many years ago, has numerous apartments that are de facto Faraday cages; the plaster walls were put up with screens. Many people complain that their WiFi has unusually limited range, and that they cannot receive cell signals. As for the idea that there's a firewall solution; it's true until one kid figures out how to VPN tunnel out, then all of them do.