i'm sorry to say it but i have never had luck getting plugs out of aluminum heads once the threads get buggard. odds are you have a chunck of aluminum stuck in the threads of the plug and its stripping the threads on the head as you turn it out.

i would probably leave the plug in there and wait until it starts running bad on that cyl before i would drop the motor for a heli-coil. you may get 50k more miles before you notice it . thats just me though, i'm lazy