If you line up the two flat sections on the stem and bolt you might be able to wedge the gap with a large flat blade screwdriver or piece of flat metal (whatever fits the gap) to allow you to unscrew.

Alternately it looks like you could grab the bolt in the middle from the bottom with pliers or needle nose vicegrips. Since the bolt doesn't work/engage, it wouldn't matter much if you marred it getting it removed.