Originally Posted by LV2TNDM

If this is the case, then the OP is going to have to detension the entire wheel and rebuild it with brass nipples. This would actually also be a solution to the stuck nipple problem. If the OP can loosen all the other nipples, this will also reduce tension on the spoke in question. Doing so incrementally around the wheel will eventually result in the stuck's nipple/spoke's tension to be low enough to break free.