“Liability,” true or not, usually works the best, IME.
It also works for strangers/random neighbors who want to use you arena or other facilities. We had a neighbor, who’s thankfully moved, who was an inexperienced horse owner who bought a 2-year-old gelding. I barely knew the guy except to know where he lived, and the few times I had brief conversations with him, I got a slightly NQR/spidey-sense vibe from him. He turned up IN our garage one day, when we had the door open (I was getting something out of my truck, turned around, and he was right behind me), and asked if he could come break his 2-year-old to ride in our round pen, since he didn’t have a round pen and was afraid he’d get hurt if he didn’t use a round pen. Ummm, NO – LIABILITY. He said, “But I’m worried about my own liability if I get hurt -without- your round pen.” I said no again, and advised him to go buy some portable panels and make his own (damned) round pen. After that, I really wondered if I needed to put a padlock on my round pen gate, but I kept an eye on it, and thankfully saw no evidence of use other than by me!
As for riding, I have two work friends who came up with the idea that since I have three horses, the three of us should go galloping through the desert. They said they’ve gone galloping many times on trail rides. Uh, yeah… not. I explained that one horse I would let them ride at a walk in the arena, one that is not completely sound that I would NOT let them ride at all, and one that is very good but is my show horse, who they could also NOT ride. So, when they came over to visit, they still thought we were taking all three horses out in the desert to ride. facepalm I put them each up on Whiskey, and they clearly didn’t know how to hold reins… He had just had his hocks done, so I didn’t want there to be any trotting that day. One of them did OK walking, with me walking at Whiskey’s side, just in case (for them, not for him, LOL). The other one took ahold of the reins and leaned WAY back when I unclipped her, Whiskey started jogging back to the barn, she kept leaning and pulling back, he kept jogging, so I grabbed the bridle and made him whoa. YEAH. SO. END of “galloping across the desert.” I like them both a lot, but was very irritated by the whole thing while still trying to be nice/friendly/make it work… “What part of no don’t you understand?” It can be hard sometimes. Sigh. But that was a year ago, and they haven’t asked again!