I would heat it up really hot with a torch to melt the lock tight. Then grab it with vice grips if enough bolt is protruding out, and turn it out.
If not,drill the middle of the soft bolt with a drill bit, then get it hot with the torch, then tap in a bolt extractor, and back it out slowly.
If you have access to a drill press, use it so that your hole is perfectly centered and straight.
Take your time.
These style work the best I've found.
http://m.harborfreight.com/12-piece-screw-extractor-set-40349.html?utm_referrer=direct%2Fnot%20provided