To make it easier, try getting two nuts that are the correct size, put them both on the thread and turn them into each other (screwing the top one clockwise and the bottom anticlockwise).
This is like using a nylock nut in other applications, but very appropriate here.
Keep turning the bottom nut anticlockwise until your bolt / threaded bar is out.
Personally I would use a third one to remind me of the height if it could fit.