You're not tightening it enough, OR, you're flooding the engine. The sure sign that the engine is flooded is if the engine won't turn over continuously with the starter. It'll turn over for part of a revolution, then lock up HARD.

Don't try to force the engine if it's flooded and hydro-locked like that. You'll bend the rod. Instead, you need to drain the excess fuel out the muffler, clear the engine, and try again. Possible causes are the tank being too high, the needle valve being open too far, and overpriming.

Still, you probably just haven't tightened the prop enough. If cutting your hand on the prop is a concern, use a rag or leather glove to hold the prop while you tighten it down. Use some muscle.

You would think that the use of a right hand (standard) thread on the engine crankshaft is a dumb move. I mean, every time you go to start the engine, you're loosening the nut right? Well, there's a method to their madness. When the engine is running, it's actually tightening the prop nut each time it fires due to inertia. If they used a left-hand thread, you wouldn't be able to keep a propeller on the engine; it'd just spin the prop nut right off!