This is not the first time I have read this....one dude had to get the Dremel out...but first:

1) Can you still get a wrench on it? If so, somehow hold the wrench on the haed then ping it with a ball pein hammer....ie smack the wrench to get some vibes going...this will help break the metal to metal "seal" from atom sharing....

2) If a wrench is useless get the old vice grips out....and lock the thing on the head tigher than tight, then ping it, then put a piece of pipe on the screw handle of the vice (hehe) grips and lever away. Careful....

3) Punt...sell the car...replace the tranny...or moto tool!