Hello GLB , I have heard of this problem before . The problem you have is that the entire shaft is spinning. This is what I would try next : Verify from underneath the lower yolk , that the center shaft is spinning when you turn the nut on top . On my bike this is where the horn is bolted on. Once verified the entire shaft is spinning , clear the area of other parts such as horn , cables , brackets to get a clean grasp of it . Then you must get a good vice-grip pliers with clean sharp teeth so you can get a substantial grip on it . This is important , you must get a firm grip on the head of the steering stem while you loosen the top nut. Get a second person to help if necessary , you don't want to slip off. If you are inclined , this shaft head can have flats cut into it so this will never be a problem to you again !
