It's the taper that holds the flywheel to the crank. The woodruf key is only for alignment.
You can try to use grinding paste to surface the two tapers but at the end of the day the flywheel is making less than 100% contact to the crank and WILL spin off again.
Does no one retorque the flywheel nut after the first run? It seems like there have been a lot of key shearing problems on the forum lately.