At What Point?

We have a family living next door to us that have a few children. When they moved in the man of the family had asked if it was okay to attach his fence to ours. Not a problem. The kids were all pretty small at the time. Last summer we began the daily ritual of throwing their balls back over the fence. I had nicely explained that if the dog is outside when the balls go over that I'd try my best to get them back in one piece. I asked them not to go into the yard if she was out. I even talked to their father about that. We have a fence for a reason.
When the snow thawed earlier this year I threw 4 balls over the fence. They did heed my advice in not coming into the yard. Since the weather has now turned nicer the balls are once again flying. They have now taken to ringing the doorbell and asking me to fetch their ball. This has now become a daily routine, sometimes two to three times a day. Once is an accident, two to three times means maybe you better take a walk to the park which is 1/2 block away and play baseball there.
The other day I caught the younger one sticking a stick through the fence and teasing Sadie. I told him that wasn't a good idea. Dad was not around as I think he works several jobs. Stay at home Mom was not around. I have yet to see her even look out the door to see what these kids are doing. A few minutes ago I answered the door and went and threw the ball back again. I suggested maybe batting the other way. "My dad doesn't want us to hit the garage" was the reply I got. Here I'm trying to nicely say "Stop this".
I had a memory flashback of when our soccer ball went into the yard behind us and the old guy came out and just took the ball. We didn't even have a chance to ask for it back. It was gone. This was a one time thing. I remember I thought he was the biggest jerk. I also recall the former neighbor yelling at my sisters once and they smeared her garage with dog pooh. I don't want my house or fence smeared by these little guys. All I want is for the ball to come over when Dad's home so he can maybe warn them. For now, if I see the balls in the yard I will return them although they may be chewed. I may be disconnecting the doorbell.