Don't be afraid to put some serious torque on it. If the nut is coming loose, it is either not getting tightened enough or the engine is backfiring and loosening it up. Both the nut and crankshaft threads should be hardened. Unless you put a 3 foot cheater bar on a 1/2" drive ratchet to tighten it, it won't strip.

Most smaller 2-strokes don't need jam nuts.