Using a rag to stop an engine causes no problems either to the engine or the prop. This used to be quite a sport with CL models if the engine had a bad run, there's no other way to stop it short of crashing. Usually though the darn thing would run out of fuel before your aim got good enough

This of course was at full throttle and not even wood, plastic or nylon props ever broke. Although at first sight it seems like a bad idea, LONG experience at doing this shows it's quite safe. The main thing is that you let go of the rag before it gets to the prop