You need to find a way to hold the pulley while turning the nut.

If the nut is spinning and the pulley shaft is moving then you need to hold the shaft.

If the nut is spinning and the pulley shaft is not then you've stripped the nut on the shaft. You'll need to cut the nut off or find an old time nutcracker to break the nut off and then replace the shaft and nut.

When you reinstall the blades use a dab of anti-seize on the threads.

The fool proof way to remove blade nuts is to use an air or electric impact wrench and an impact rated 6 point socket. Brute force and a big wrench will get you into trouble more times than not.