first, make sure that the allen key you are using is not at all rounded. If it is, get a new one, or grind the end down a bit so the edge is square. But I guess that it is too late for that. Give it a bit of CRC or something. The, get a hack saw out and delicately cut a grove in the head so that you can use a flat head screw driver. If you still aren't having any luck, there are a few other tricks you can try. A bit of heat (mini gas torch) often makes it a bit easier to release stuck bolts. If nothing else works, you can drill it out, but generally, if you haven't done it before, and you don't have the gear, it is much easier to get someone else to do. A decent bike shop should be able to drill it out, or you could take it to an engineering workshop.
nygeo said:my Avid (arch rival supreme) rim brake has a stripped bolt that needs to be removed. It looks very similar to a crank bolt perhaps 3 or 4mm allen. What are some good ways of releasing it?