When you mount the prop, set it so that when you rotate the prop forwards, you start to feel compression when the prop is level. When the engine stops, the air pushing against the prop will do exactly that, push the prop foward until it hits compression and can't rotate any more.

Fwiw, I often land on grass with the engine running, or the prop vertical, and rarely do I break a prop doing it. The trick is to touch down as gently as possble, nose high. If you touch smoothly enough, the prop will get pushed out of the way instead of breaking. (but I do set my props to be horizontal most of the time anyway)