I'm a little confused here...are you saying that with no prop the nut is so tight the crank turns? Is this a locking nut (nylock or similar)? If it's just an ordinary nut then you've got a big thread problem (or Loctite ).
Try using a normal spanner on the nut but treat the spanner like a prop when hand starting..in other words give it a sharp jerk against compression. This should move even a locking nut bit by bit. You could also deliberately hydraulic lock the engine (inverted of course) but if you're not too sure about the relative strength of rods etc I wouldn't really advise this.