My dog is well trained and perfectly relaxed in public but if anyone tries to pet him he reacts with aggression. He won't do this with people he knows. Is there a solution or is it better to respect his nature and not let strangers approach.?

Hello Cindy.
This is about a 3-year-old, male border collie dog, it's not a fearful dog, He is confident and he has advanced obedience training, and also agility training. He can walk among people in malls, markets, and massive events keeping quiet and relaxed but if a stranger whether a woman, man, adult, or kid, approaches and tries to pet him, he reacts with aggression, even if people only praise him from a distance. He won't do it with people he knows.
I would like the dog to stop reacting like that, is there a solution or would it be better to respect his nature and just not let strangers approach him?