Okay, I've read the whole thread. I wanted to be sure I am not misunderstanding things here. What this boils down to is what are the rules in THIS public barn? If the rules say no studs or even no loose horses in the arena, then Ropo absolutely is entitled to be upset. If it does not, then the stallion had every right to be there. His reaction scared her. Understandable. What everyone needs to remember is that ANY horse can react exactly the same way. I've had geldings that would stomp you if you walked into their turnout pen yet were perfect gentlemen and child-safe the rest of the time. Horses who have been stalled come out of that stall with a LOT of pent-up energy. They are perfectly behaved all the way to the turnout and all the way back. But once inside they know they get to jump and buck and run and kick and roll and do all the horsey stuff they aren't allowed to do all the rest of the time. Freedom! It really does sound like this is what happened here. And since Repo was a stranger, the animal knew she wasn't there to take him back to his stall, so he challenged her!
To me the issue really isn't the horse. The issue is what are the rules? If the gal who owns him was within the rules, then the most anyone can reasonably expect is a note, flag, or something similar that says the arena is in use. That's an issue to take up with management.
FTIW I have kept, trained, and shown stallions for most of my adult life. Most who are handled from birth by a responsible trainer have no more issues than anybody else. Mares can be just as cantankerous as any stallion, yet nobody thinks twice about keeping mares. IMO way too many stallions get a bum rap because of people's misconceptions and not because they've actually done anything wrong. It's like assuming that all pits are gonna eat your kids.
JMO
Rusty