Take the left hand thread removal bolt out. Smack the head with a big hammer. The force and vibes from the right amount of force will break loose the threads. I have used heat as well before ad that is my last resort. I normally use what I just described or, like Wade said, a nice dimple from a sharp punch/chisel blade and then keep smacking it in an unthread direction will do it as well and is typically my second attempt choice. If it has threads it will eventually turn out of there. It may bring some of the case threads with it though. Inspect the case threads and clean THOROUGHLY. On a component like that I like to just dab grease on the threads of the new plug and thread in.
