I take a punch and hammer and put the punch on the outer edge of the plug and get the one side to push inward, once the plug starts to come out on the other side I use a vise grip to pull the plug out of the hole. Sometimes a small piece of wood helps to give the vice grip something to pry against. Take your time and don't hit to hard or the plug will fall inside the block. If that does happen get a magnet or piece of wire to get it back up to the opening and the grab it with a vice grip to pull it out.
