Threats

Yesterday evening my long-time tennis partner and I were out playing, and two teenaged boys were on the court next to ours.
They were not taking their game very seriously, and they hit one ball onto our court, making us replay a point. Then another one.
The third time they did it, I said sternly, "Boys. Do that ONE MORE TIME and you are going to be our ball boys."

They looked scared to death. It did the trick.