Wall tiles that are mounted on plasterboard present a dilemma when you decide to remove them: The force it takes to hammer or pry off the tiles will usually be too much force for the wall, damaging it. The solution is to go about the project more delicately, first extracting the grout with a grout saw (a square carbide blade on a handle), and then easing the tiles off the wall.
Set the flat edge of your grout saw against the surface of the grout next to a tile that you want to remove. Press the blade firmly into the grout, forward and back, digging into the grout. Get out as much as you can, all along each side of the tile.
Repeat the process for each of the tiles that you want to remove.
Hold the sharp end of your chisel against the base of one tile you want to remove, inside the extracted grout line. Tap the back of the chisel with a hammer, working the tip under the tile. Don't try to pry out the tile, but rather move the chisel around to different points along the tile, working the tip under it, until you break the bond with the wall and the tile falls out.
Repeat for each tile you want to extract.
Clean up the residual mortar or tile adhesive from the wall with the metal putty knife.
Wear goggles when removing tiles.