Heat. If you heat up the bolt it will singe the surrounding wood and become easier to remove. Not too sure how you will go with removing the rest of the nuts though. Some WD40? and a bit more heat perhaps ...
Oh, and to apply heat, get one of those cheap soldering irons, plug it in, and hold it against the end of the bolt. Alternately, get a battery charger and clip a crocodile to each end of the bolt, but not for very long as it heats up remarkably quickly!