The last time I successfully removed a seized stem, this was my procedure:

1. Attempted the standard technique of loosening the fixing bolt and tapping it with a hammer
2. Put the handlebars back in the stem and an old junk wheel in the fork and twisted with as much force as I could muster
3. Removed the fixing bolt and sprayed a ton of PB Blaster down the hole
4. Sprayed a ton of PB Blaster around the headset
5. Let it soak for a day and tried brute force again
6. Soaked everything with ammonia overnight and tried brute force
7. Heated the whole thing with a hair dryer and tried brute force
8. Cooled the whole thing with compressed air and tried brute force
9. Beat the stem severely with a 3-pound hammer
10. Gave up and assembled the bike with the stem in place
11. Rode the bike for about 500 miles
12. One day noticed that my handlebars had become misaligned
13. Loosened the fixing bolt and gave it a light tap
14. Pulled the stem out with ease

YMMV