Plumbers use two methods to remove the clean-out plug. The first is an old-fashioned steel chisel and a ball peen hammer on the face of the clean-out at a 45-degree pointing in the desired turning direction (counter-clockwise). Once there's a little divot in the clean-out, turn the chisel on a sharper angle to provide more turning force. If that doesn't work, chisel right through the surface of the brass plug or cast plug and cut the rest out with a sawzall. Once the center of the clean-out out is removed, a heavy screw driver will pry out the threads.