He's got this from a friend who doesn't like her toys being played with by other children and now he's using it for every situation he doesn't like. ie.
Mummy is chatting, he shouts stop it at the person I am talking to.
He doesn't want to eat his dinner, he shouts stop it at the plate.
A child he doesn't know sits near him and he tells them to stop it.
Ladies talking too loudly in cafe, he shouts stop it 
When people laugh and he is not in on the joke
So far I have said, "thats not kind, we don't tell people to stop it" type things but it continues. It's annoying more than anything because it makes him sound like a grouch when really he is not! Is this a phase? When will he stop it! 