You have a quill stem. This is what you're trying to separate:


See the bolt sticking out? That's the top bolt you unscrewed. Hitting it transfers the force to the wedge, which gets it unpinned from the inside of the fork and the stem. The large nut is threaded to the fork and holds it in the fork tube: If you remove it, the stem will still be inside the fork.

I've always used flat-faced metal hammers, and normally it takes just a tap.