Put food in bowl, stand over bowl facing the bowl with your back to the horse, if that horse aproaches within 3 feet of you , yell and smack them hard in the chest with a crop , then turn back around and stand over the bowl. Play with the food shake the bucket, what ever, if that horse comes up behind you, repeat the yell and smack. Once the horse is keeping his distance, simply walk away. Don't call the horse, just completely ignore him and walk away.
Put a flake of hay out in a field, and you will see the exact same behavior from the herd boss. I've havent seen many horses take more than a couple lessons to figure this out.