try lunging her in a round yard. Ask her to slow down, and if she doesnt, you can 'corner' her by bringing the lunge whip in front of her and forcing her against the fence (hard to explain over the net). As soon as she stops, take the whip away and let her keep walking foward. If she tries to turn and go the other direction don't let her, keep her going the same direction. She'll soon learn that when you say stop, you mean stop.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare