The problem is that the nut holding the prop is bottomed out against the base of the threads on the shaft or the drive washer face is worn smooth. Unfortunately, grabbing the drive washer with a pair of pliers is probably the only way to get the nut off. This is ultimate tacky way to do it. The drive washer will have to be replaced when you get done.

DON'T EVER USE A PISTON STOPPER ON ANY MODEL ENGINE!!!!!
It's a clever invention designed to cause damage to your piston.

If you can't get this handled yourself, send it to YS Service and we will get it apart.

Good Luck

Dave