As a junior I would always take headshots at guys like this when they came to the net. Then when they made the error, I would say the same thing they were saying (yes..cmon..whatever it was). That always worked. Coming from a football background, I found most tennis players in the juniors to be pretty easy to screw with. The toughest guys were all on my team..we were all multisport guys who played football and baseball for years so we had dealt with everything by age 10 or so.