Well put, Saddlebag! A chain is not the answer! If you are lunging and your horse does not want to stop, then let him keep going forward, but make it your idea. Ask him to move on by clucking to show that you are the one saying keep moving. Then ask for a stop again, if he does want to stop, ask him to move on with a little more energy. The idea that you are trying to get across to your horse is that if you don't listen to me, you are going to have to work harder. When you ask for the stop use the same cues every time, like a soothing voice and a relaxed body. When he stops let him rest to show that he did the right thing.
Do you know how to get him to change directions while on the line? Changing directions frequently keeps his attention on you.
More force is never the answer. Work on the training, because Saddlebag is so right, you need your horse to respect you, not fear pain, in order to have a safe, reliable horse.
You don't have to be crazy to work in a barn, they'll train you!