I'm inclined to think the bolt is bent, as you say you can turn it a little. It it were me, I'd remove the engine and drill the bolt out in steps. An AN5 bolt is only 5/16" diameter: start off with a 1/8" pilot drill and work your way up - 5/32", 3/16", 9/64" etc, keeping the speed down as the drills get bigger.
I really wouldn't hammer it out... damage could be expensive to rectify.