If in doubt, you can stop the shaft turning with a bit of light line with a loop on one end. Snag one of the coupling bolts with the loop and make the line fast to something strong nearby.
Make the line light enough so that if you forget it or need the engine
in a hurry it will snap without doing any damage to the shaft, coupling or bearings.
I used this setup for years without problems. (No longer needed thanks to my feathering kiwiprop.)