I trust you've only undone the bolt a few turns, so there's still maximum thread trying to push the cone or wedge down, not just bouncing the bolt end up and down on its top? And used a soft mallet or some wood between hammer and bolt? Has the wedge or cone fallen out and the stem remained stuck?
After that, try plus gas or similar, dribbled in alternately down the stem and up the forks with the bike stood in a trough... then you get onto really nasty stuff.