First off there have been mentioned of documenting it, and you should definitely do so. I would include calling animal control in this so there is a trend if something does happen.
In terms of protection though, there is something called pet corrector spray. It’s the equivalent of an air horn for dogs, and is directional. Humans only hear a hiss. Might be worth an investment there.
When I’ve encountered off leash dogs I put myself between the dog and my dog. Between that and the corrector spray it should be a pretty convincing argument to the little shit to stay away.