I used a simple pin that you could slip a small padlock onto to lock the ball to the receiver.  Had another locking pin for when the trailer was on the ball.
If you were on  the BI you could have them..

I already bought the Curt locking hitch pin.  The set for the channel hitch includes that one plus one to replace one of the other two pins that connect the adjustable ball mount to the hitch.  Of course, someone could remove the third pin, but you're less likely to have a problem towing a small trailer.

This is the one I have now:


https://www.curtmfg.com/part/23518


And this is the set that includes the one above and another for the channel mount:


https://www.curtmfg.com/part/23556


From videos I've seen, these barrel style locking pins are easy to defeat with a length of pipe over the locking end.  A bit of leverage, and the lock pops off.  I'm more interested in casual vandals thinking it's funny to sabotage a trailer hooked up to a tow vehicle.

I've been imagining some way to prevent a pipe from gaining a purchase on the lock, but so far nothing that might work well without welding.

Put a Do Not Remove Hitch sign on it.  That will work as well as a Gun Free Zone sign.  All law abiding people will respect your sign.