The shower escutcheon is the protective plate that keeps the shower knob and valve from being damaged and also separates them from the pipework behind the wall. The shower escutcheon is often very difficult to remove, as you will have to take off most of the showerhead first, and then remove it from the wall.
Step 1 - Take off the Shower Knob
Once you have shut off the water to the bathroom, remove the shower knob and the faucet handle by unscrewing the parts. You will need to unscrew the long bolt screw that holds the handle into the valve. Place the front of the showerhead and the handle in a safe place.
Step 2 - Remove the Shower Escutcheon
You should now be able to see the shower escutcheon. This large plate will cover the hole just before the valve. Its purpose is to protect the valve and hold the shower knob and valve handle onto the wall, and it is often caulked, grouted, or plastered into position. In order to remove the escutcheon, take your chisel, and place it above the plate. Using your hammer, gently tap the chisel into the grout which is holding it in place. After tapping all the way around the escutcheon, pry it off the wall using the chisel.