Cut two disks of sandpaper about 1 inch in diameter and glue them paper to paper side. Cut a hole in the center of this the size of your crankshaft. Put this sandpaper disk between the spinner backplate and the prop hub. Now you can tighten the nut down without the prop or backplate slipping. Works for me every time.

I usually bear down pretty hard with a mid-sized adjustable wrench when I tighten the nut. The only one I ever stripped out was a brass heavy hub. Good luck!