get an adjustable wrench, put it on the bolt and hit the adjustable wrench with a hammer in the correct dirction. This will jostle it loose. remember the threads are in the opposite direction. (I forgot the dirstion a few weeks ago when i was putting a new timing belt in... worst part is i've removed the fan before and told people that it was the wrong direction.... sometimes... man)
by the way, just hit the adjustable with all you have in you... don't be gentle. (well don't break anything but....)