If you can't get a MAPP gas torch (they are $30-$40 at home depot w/MAPP gas) then try TIGHTENING the bolt slightly. I know it sounds crazy, but it was a trick my dad taught me and it works. Sometimes this will break the threads loose. If you get it to move at all by tightening it stop immediatley and try to loosen it. If you got it to move at all, it should break free.
The other trick he taught me was to pound on the head of the bolt with a hammer. Be careful not to scre up the head of the bolt. This can also jolt the threads loose but works less often for me than the tightening method.