Get a better fence charger.We have horses/mules every spring that get put on the property next to us, they push against the fence and we were repairing it,but they, especially the mules would push it again until the posts were loose.So, we got a POWERFUL charger and put up electric fence.It was actually comical to watch the mules walk up and start to lean against the fence, then they hit the wire. You have never seen mules move so fast. Then the next horse walks up to the fence and touch his muzzle to the fence.... Man, you could hear the snap when it got him. Never had them push the fence again.
Call animal control,complain about the horses in writing, get a good fence charger and put it up. Snap and the horses won't break into your pasture anymore.