I would agree with the use of a 2-3″ wide strap looped over the end of the blade and tied off to the motor bridge handrail or some other visible place. As an added equipment safety measure a LOTO tag should be applied to the strap as part of the LOTO to ensure that the strap is removed before the fan is returned to service.
I’ve also witnessed some facilities utilize a large, 2 piece, pipe clamp, with extended length that is bolted to the motor drive shaft after the guards are removed. The extended length of the pipe clamp would then contact the yoke area on the motor (as applicable).
Not all fan motor/gearbox will have this sort of access so the easiest and quickest means of securing fan blades