Can you get the stem itself out?
Take the stem out. Give the wedge a good hosing with your favorite lubricant. Let set. Thread the bolt back in a whack it again.
The wedge may also be jammed against the bottom of the steerer where it necks down. Flip the bike over and attack from the back side.
This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.