Heat it to break any thread sealer... then drill them with a left handed drill bit. Most likely... it will bite, and un thread. If they don't... then use a stud extractor, and remove. OR... drill them out completely, and run a tap to clear the threads.
Try the largest star bit you can get in the and use a small impact driver (harbor freight has a cheap one). I know from experience these are $3 each fro boats.net
