i had the, same thing happen to me before what you do is if you have a dremel use, the cut off wheel and make a flat line on the front of the screw so you can use a flat head to unscrew it. and make sure you dont hit the teeth on the bell with the cut off wheel.
also i would, get a hex tool its much easier and when you tighten the clutch bell screw push in while you are turning it.