Maybe start with a simple fact: their chickens are regularly eating from your feeders. Yes, that sort-of counts as foraging their own food, but it's probably not what the neighbors had in mind.
And, since you don't want to get animal control involved, consider keeping your own chickens (and their feeder) shut in for a few weeks. No, it won't be nice to the neighbors' starving chickens, but if you cut off that supply, maybe the neighbors will see that their chickens really do need more feed.
You say they're giving two cups of food daily. How many chickens do they have?