a few months ago, i shouted "no!" really loudly at a little boy in the park near us as he had some blower-type toy, and he ran up to DS and blew in his face with it (you push a pump and it poots air out), DS thought it was funny, so the boy tried it on my springer, and i just reacted
i explained to him that he wasn't in trouble, as he was only being friendly, but that he shouldn't take such a risk, even with a friendly dog like mine
i didn't even think, as you never know how the dog will react