If you cannot remove the fork, there is on other (destructive) alternative I have used: Saw off the faucet from above using a sawzall and a fresh metal cutting blade, leaving about 1/2" of the faucet.
I had to do this on a frozen faucet set. By cutting the top off, I was able to turn the remainder of the nut on top by gripping it with vise grips.
Be warned: This is a last ditch tactic and not for the faint of heart.