Do your neighbors have pets? Dogs, anything? (surely they do, or why would they have dog food, eh?) Suppose you go visit them with a dozen eggs or some cookies and an offer to chat. Explain that your chickens, like their dog, are on a very specific, scientifically formulated diet to get the healthiest and best eggs from them. And giving your chickens dog food makes as much sense as you giving their dog chicken food. Or for people to eat dog food, see? So silly! So you know they will understand you asking them not to do it any more. And hope they enjoy the eggs. Thanks for being such good neighbors, hand shakes all around. Problem solved, one hopes. Good luck.