I often come across the "Don't worry he's friendly!" people and I also say like
@katiemcat that my dog is not friendly. My terrier mix was a reactive mess when I adopted him and he's still not perfect. He used to shriek bark when he saw other dogs outside of our home and would even attempt to bite his leash or me out of frustration. He's a lot better now, but that's because with lots of training we've built trust and I know his triggers. People who let their untrained dogs do whatever at places where dogs are not allowed off leash genuinely upset me - because my dog and I have worked so hard and he should be able to enjoy himself without worrying about his safety.
After I tell people that my dog isn't friendly, I tell them that their dog's behavior is their responsibility. If my dog were to bite their dog and their dog was the one off-leash approaching mine without permission, that's on them. I really emphasize how it could cause a dangerous situation for both the dogs and humans involved. I also tell them that it is incredibly irresponsible and rude of them to allow their dog to greet anyone- some people are afraid of dogs, some people don't like them, some dogs are aggressive, etc. A park with an on-leash rule is for everyone to enjoy, not just the irresponsible owner with the untrained dog. I think mentioning both the safety of their dog + calling out their irresponsibility works. A lot of people are just oblivious
You could also get one of those "Do Not Pet" collars for your dog to help send the message that your dog is in training or aggressive. I use a gentle leader with my terrier and that often makes people think twice about letting their dog come near mine. If I take my dog to a place where a lot of dogs are, I do have him wear a Baskerville muzzle. I don't think he'll bite, but it's not something I'm going to risk