The traditional answer is to follow traditional stripped screw advice. Use super epoxy (very carefully) to adhere a new surface that you can loosen the screw with. The trick is to make sure you use enough to give you the necessary adhesion, but not so much that it adheres to the case in which case you are royally screwed. (Pun not originally intended.)
This link has a number of other suggestions as well, though not all of them would work well on a lens. It might also be worth simply cutting your losses and sending it in to be repaired by a Canon tech before more damage occurs. It'll cost more than if you succeed at removing it, but be a lot cheaper than if you have to send it in after failing.