First, balance your prop. That alone might do the trick.

If it doesn't, use the old safety wire trick. Get some thin copper or steel wire, wrap it around the muffler at the pressure nipple with one wrap around the nipple. Now twist the two ends of the wire and wrap the twisted section once around the header opposite the way the muffler screws off. Finally, tie it off at the head. Now it can't unscrew without tightening the wire.

Do a Google image search for Safety Wire for pics