You need safety even if you intend to school the horse. Get the owner of the horse to front up and fence an area off at his/her expence so it is contained. Since it is free grazing.
It has a large area to move in but choses to be around the house for company however it does not know how to act and is treating you as a lesser member of the herd.
If you are going to work with it try hitching it to a post for a few hours at a time each day getting longer and longer untill its tied all day. Its not cruel but can quiten it down considerably, but it needs to be in a managable paddock.
I take it at times you do not move to quick, nor do I. I have learnt having a 4 foot firm riding crop or the such like, easy to carry and if the horse gets in your space just lightly tap it on the side of its face 2 inches below the cheek towards the nose and it will turn its head away and eventually it will move its front feet away as well which is what you want. It will learn not to croud you. However get it into a paddock for safety.