Taking Apart Roadmaster Falcon 2 Tow Bar

First I would try holding the round bar with a strap wrench. If it slips when trying to remove the nut, then wrap the round bar with some tape, like Gorilla tape with enough thicknesses to keep the pipe wrench teeth from going through the tape to damage the round bar. You may find that once you can hold the round bar from turning, you may still need to use an impact tool, either a manual one hit with a hammer or a pneumatic or electric one.

As a last resort, I keep a nut spliter tool, in my tool chest. You do have to replace the nut with a new one, but the old one, will come off, usually without damaging the threads on the bolt.