I know a mare who used to do that Constantly when she was lunged. It can be Very annoying. You need to shorten the lungeline and once she begins to slow and turn pitch the rope at him. When he is calmly walking in a circle a couple feet away from you let the rope out another foot and do the same thing as before and when he gets out of your reach of the rope then when he faces you give the lunge a tight pull and say NO very firmly and get a couple feet from him and don't use your hand to turn him, pitch the end of the lungeline at him. When I say pitch the rope at him I do not mean hit him with it, NEVER do that, just throw it through the air towards him. Repeat this and it will take a while but once he goes in a couple circles at a fair distance away from you in either direction call it quits for the day. Repeat it as often as possible. Don't let him get away with it. He will eventually start going faster to think he can outrun the rope then stop and face you, but when he begins going faster you need to MAKE him stop, face you, and ask him to move off.