I brought a horse home on trial last December who had a lot of issues with ground manners. With her, it seemed more like a lack of training than actual aggression, but it was still quite dangerous, as she had no respect for personal space and I couldn't longe her at all without her trying to barrel into me.
The trainer who I had evaluate her had a technique which was very effective. When the horse would try to come in on her, she'd do a little leap right at the horse, extend her arms up and let out a big "PSSSSH!" sound- basically answering the challenge by becoming larger and more threatening. It was pretty funny to watch, but the horse definitely took note of it.