Just ask them to stop. I don't really see this as a property issue, its yours and if you wanted to you can stop them from using it.
Some easements are "rights", off the top of my head I think that mostly is utility companies.
Back when I was a kid I used to cut a lot of lawns and solved this problem by just picking up the neighbors as another account.
If the fence is right on the line I don't understand why they are running the mower over their, they should just walk it with a trimmer. They probably don't know its your yard.