My favorite suggestion I've seen on BYC has been:
Catch the dog and chain it up on YOUR property!
* When the owner comes looking for it, you can tell them you see that they're able to care for their dog except do the part of appropriately controlling it, so you're doing that part for them.
* Do not feed the dog--Tell the owner that if they want to keep feeding it, they'll need to also do the restraining part on their property so it is accessible to them for feeding.
* If the owners don't agree to change their dog-restraint practices
pronto, turn in the dog to Animal Control.
I prefer to directly resolve things with people where it's reasonably possible to be safe & effective. Then I don't have to have an outside entity handle the situation while I don't discuss it with the person and end up walking around on eggshells around them afterwards.
I think this method could allow you to have direct interaction while being somewhat humorously good-natured, yet make your point strongly, powerfully and hopefully with good effect.
Disclaimer: I have not tried this method, so I can offer no guarantee regarding its effectiveness.