Don't you just love it when stuff like this happens?

Seems to be a common thing when you thread an aluminum nut onto a steel prop shaft. The aluminum galls a bit, and binds up on the threads. Whatever..

I've always just twisted the prop a bit to put some friction on the drive washer and that seems to allow the nut to be removed.

Alternately, you might try heating the nut a bit with a monokote heat gun to free it up.

Good luck