I agree, it probably is seized into the head. That's the action of dissimilar metal when exposed to weather and heat. Jarring it is the best way to remove. An impact gun sometimes works. Heating the head and not the plug will expand the hole. There is (to the best of my knowledge) nothing that is guarantied to work. When you do get it out, the threads will likely be damaged. But this can be repaired with a Healey coil (spelling my be off).