Using a Pry Bar
If you have a collection of home improvement tools, then you will likely have a pry bar somewhere in the tool kit. These tools are used in a lot of demolishing work, and can also be used to move stones, lift off drywalls, and do any kind of removal work where you need to pull something from something else.
Step 1 - Insert the Pry Bar
Look at the part of the wall that you need to extract. Take the curved end of the pry bar, and slide it behind the wall. You may need to push it in at an angle, and then move the handle around until you have the pry bar at right angles to the wall. You can begin by having the pry bar parallel to the wall, with the edge of the curved end in the hole in the wall, and slowly move it around until it is in the right-angled position.
Step 2 - Use the Pry Bar
With the handle at right-angles to the wall, push the bar slowly around until it is parallel with the wall. You should be standing with your left arm against the wall, pushing the pry bar away from you. The wall should then come away, and you can remove the pry bar and insert it again, and repeat.