Last summer I ran into the very same challenge and solved the problem with a chisel. Now before you freak out... read on... Use the sharp edge of the chisel to bite into the head of the bolt and angle it to turn the bolt out when you hit it with a hammer. This will crack it lose and then finger thread it out. Use the chisel to set it to tight when reinstalling it. If you can look closely at the attached picture, you can see the chisel mark in the head of the bolt made previously.
