Last year sometime my yearling started to get really aggressive with her grain. I usually lead her to it, and unclip her and let her go. She was starting to pin her ears, pull me, and was starting to turn her butt to me if I approached her bucket. What I did with her was I stood in front of/in the bucket (one of those big, black, round and low ones) and stomped. I pushed her back, waved my arms and the like. When she tried to get the grain, I would drive her back. I essentially acted like one of the boss horses, like you see them do in the field.
It sounds crazy, and it looks crazy, but it worked like a charm. I did that only once, it was about a minute and a half. Now I can approach her whenever and she'll back off the food, and waits for me to get out of her space before she eats. This may have worked partly too because she was so young at the time.