Internally corroded stems can be a pain.

No one's mentioned this, and it's pretty obvious, but just in case: Turn the bike upside down. Spray in your favorite penetrant from through the fork crown below. Use a lot. That should free up the stem bolt. You can use a fair amount of torque to do this.

I don't really like the brute force methods. You're likely to damage stuff. It's not how it would be done in most bike shops.

Sometimes it's helpful to tap the stem out from below. Good idea to remove the front brake.

If all else fails, heat and cold will free up pretty much anything. But that method has its own hazards so I'm not going to describe it.