I think it’s important to phrase this issue in terms that would resonate with your neighbors. Are they trying to save money by having the girls free range? If so, then raising the point that their chickens are “robbing” you might be something that they can understand. You can take a video of them gobbling down your feed and say something along the lines of, “Look, I really like your chickens and all, but they’re getting into my run and eating our feed. I’m not sure where else they’ve been able to forage, but they’ve decided that my feeders are the best place to get a meal and they’re eating a lot. I worry what will happen if your whole flock discovers where these few have been going. I don’t mind feeding them, but would you mind reimbursing me for the cost of the food?”
If they aren’t willing to pay you back, then you could suggest that they need to come up with a better way to keep their chickens out of your feed. You can always play a little dumb and say, “I don’t know if it would help, but (insert something about spreading the feed out so all chickens can get to it equally, trying to feed them more just to see if it would keep them closer to home, etc)
If you have a good relationship with them, then tell them that! “Hey, I’m really glad that you guys are our neighbors and that we’ve been able to get along so well. I don’t want chicken feed to get in the way of that, but at the same time, we can’t afford to feed your flock and ours.”
Just some thoughts, good luck! That’s a tough situation to be in.