What is appropriate and courteous horse ownership?

I agree with jade. But what i would do if i was in your predicament, i would probably use the horse manure for my flowers, because plants love it. I grew up around horses, and we always used their manure for our gardens. And do you live in the city, or town? Why would they have horses in town anyways? They should know things like this happen. But just like Jade said, report them or something. But if i were you, don't put any harm on the horses, it's not their fault, it's the owners, because their not watching them, and probably don't feed them enough. So the horses get desperate and do things their not supposed to be doing. What kind of fencing is it tho? If it barbed wire, horses can just go over that. They can't really go over electric fence tho. If anything just put privacy fence up. I can't really tell you what to do, because this has never happened to me. But what i can say, is take pictures of when the horses are doing it, and report the owners. Hope i helped! :)