Sometimes the plug gets stuck pretty hard in the radiator and removal will damage the radiator. Just pull the lower rad hose and you will do the same thing.
Even though the Plug has a rib/tab down the middle making you think it is threaded into the radiator...in reality it is just a molded plug with rubber o-rings that pushes straight in and pulls straight out. So to remove it (although it is probably better to just remove the hose) just grasp the rib and pull straight out while maybe twisting back and forth.