Is the stem still stuck in the fork or is just the wedge shaped nut stuck? If just the wedge shaped nut you should be able to remove the fork from the frame. With the stem and headset parts gone then you can look at how the wedge-nut is stuck. You can then determine the best way to "tap" the offender out of its hiding place.
If the stem is stuck, I'd use PB blaster (which, IMO, works better than WD-40) to soak the whole works for a few days applying at least once a day. Then with the wheel between the knees and the bars ends in both hands, wrench that beast off! Best of luck!
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Bob
Enjoying the GA coast all year long!
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