How to tell dog owners you dont want them near you without them getting angry

I dont want strange dogs coming up to me, how would I tell the owners without them getting aggressive?All these owners are like "he is harmless", but all owners say that after they attack someone. I dont want them near me. How do I ask them not to come near without them feeling like I am saying their dog is uncontrollable?

I asked a guy today to keep his dog back and he got all weird and defensive and saying his dog is harmless but I dont think asking for the dog to be kept on a short lead is that much to ask. How do you ask them without them getting all defensive?

I dont have a dog myself, I just dont trust them after all the news stories