Paint or wallpaper that blocks signals are perfectly legal, so I'm not sure what you mean by "illegal to block wifi/cellular signals".
You can't transmit and jam others' signals, but that's different.
You can't transmit and jam others' signals, but that's different.
In the eyes of the FCC it is no different if it is willful.
Nobody has ever been prosecuted for their choice of paint on their private property.
That said, I don't think anyone has ever attempted a ruling on whether chicken wire/Faraday cage in the walls of public spaces is permissible according to the FCC. I'd doubt such an attempt would make it far; there are so many buildings made with metal siding which do a fantastic job of reducing cellular signals to useless levels.