Ok so is the screw stripped or is the allen key stripped? Never heard of the screw stripping after the fin is in. Try another allen key, I think the future ones hold up a little better than the FCS ones. OR just get one from a hardware store. good luck.
buy a set of screw / bolt extractors from you auto parts store they are a round 10 bucks for a cheap set but will get the job done for ya or you can use a left handed drill bit and hope it bites it and unscrews it as u are drilling but they cost a bit more
I know for a fact that they make the FCS key out of a softer metal alloy than the set screws for that exact reason. You will strip the key before the set screw if you really had cranked down on it. Try a new key, and if that doesn't work and the set screw is damaged I would try using a slightly larger Allen key to get a better grip on the inside of the set screw.