Use a strap wrench to hold the ball while tightening the nut. Once you have some good torque to it, you can tighten the rest of the way without it spinning. Definitely 3/4” territory here.
I've done this, and it worked pretty well; once it's as tight as you can get it with a strap wrench, it's tight enough to use an impact if you have one, or crank on it with a big ugly wrench.
My Dad used to take a cold chisel and bung up the exposed threads so that even if it got loose somehow the nut couldn't spin off. You don't always have exposed threads, of course, but you could also use a large punch or chisel to upset the threads at the edge.
I'm not sure this extra effort is really needed. I don't know if I've ever heard of a tightened hitch ball getting loose, but then again maybe it has happened.