Careful with putting anything in to stop the piston. It is very easy with the soft metals to ruin your piston/sleeve. What I do is rotate the crank by hand counterclockwise until right when it starts to pinch. Then pull out the glow plug, fill up with afterrun oil , and reinstall the glow plug. Now the piston should be hydro locked and you should be able to remove the clutch nut. If it takes a lot of force (if you did something silly like use red Loctite), you may end up damaging the piston rod, crank pin, etc. If you notice excess force will be needed, stop. You may have to heat up the clutch nut to free it up a bit.
...and +1 on what Grinder said about collets.