I had a game where this Sivir was harassing a Brand cause everytime she jumped into a fight (normally at a disadvantage) Brand wouldn't follow suit, I understood Brand cause he is squishy as hell, so he had to be careful and he wasn't having the greatest of games to begin with, but nothing really that bad. I got fed up and told Sivir to shut up cause it didn't help having her bitçhing all day long, only to have her saying "oh well, you're the two negative (KDA)", I was a 2/4/21 support Taric at that moment.
I've taken to simply say "You're right! I did just Kill Secure your target! You sir/ma'am are welcome! You can free farm a full champ kill worth of minions now and take their turret!"
I then explain how a minion wave is roughly the same as a champ kill and waaay more important. Throw some math, make a snide remark about him/her being a kid and usually the can it.