It had a metal band connected to it at one time. That band easily bends. I would suggest hitting it with PBlaster or similar and trying to fish a wrench into the hole. If you can get a wrench on it the frame will hold it in place. Alternately if it's not frozen on you might be able to jam something between the nut and wall to keep it from spinning.
If that doesn't work and you can loosen it enough, a sawsall will cut the bolt off.