tramsay, what you have to do is sacrifice the alum nut. What is happening is that the thread in the alum is breaking loose, ie the thread is stripping, and any effort on your part to undo it is only causing the stripped thread to roll up on the shaft and thereby causing it to jam harder. You will have to cut the nut loose, by using a hacksaw to cut diagonally across the nut, being extremely careful not to cut into the shaft thread. Then prise a screw driver into the slot to spread the nut enough to allow it to be undone. Then by using some means to hold the shaft , by going in through the back plate, you should be able to get the now wasted nut loose. Remember you need to be able to allow the nut to expand to stop the roll of alum jamming the shaft thread.
As for tdubya's problem, beg , borrow, or purchase 2 nuts of the same size as your shaft, screw one nut onto the shaft up to a full thread from you prop nut , the screw the second nut up to the nut you just fitted,and tightenthese two nuts. Placing a spanner on the outer nut, try undoing the prop nut. If the 2 nuts are too wide, the file them down in size by removing a third of their depth, and refit as described above. Just remember to only hold the outer nut with the spanner while trying to undo the prop nut.
Bill , a retired engineer and model pilot