Faraday Fabric,Faraday Cloth 43.3"x118" Big 3.3 Yard Military Grade Conductive Material Protection EMP & Signal Blocking Fabric from WiFi,GPS

Unfortunately, in the US it's illegal to block cell phone and wifi signals using signal jammers. Prisons have this problem all the time. But you could use Faraday wall paper: https://www.amazon.com/Faraday-Conductive-Military-Material-Protective/dp/B0CZP2QGS6, but good luck getting Facilities to install it.