I am assuming from your description that you have what is often called a "quill" stem. If so, this is what you do:

Loosen slightly with 6mm. Tap with a hammer. Lift out.

If it is still not coming out, then grasp the bars holding the front wheel between your legs and rotate the bars back and forth while pulling up. Think "pulling a cork out of a champagne bottle".

If your bars are still not rotating relative to the wheel, then you might have aluminum oxidation that has "frozen" the stem into the fork. This is not good. It could involve a bit of destructive removal (stem destruction only).