Thanks for all of the suggestions so far. I feel a bit stupid, because I know I shouldn't have been using a ball-end wrench, but the correct hex was out in my garage, and I was a bit too lazy to just go up and get it.

Lesson learned.

As suggested, I did try tapping various sides with a punch and could pop one side up at a time, but never enough for it to come out or even have enough to grab with a needlenose.

Also, I can't cut a slot across the bolt with a dremel, because the bolt is recessed in the stem too far.

Perhaps I'll give the heat expansion method a try.