As others have said, it sounds like he is simply confused. First off be sure you have his working level. You can teach him quite easily how to turn it off by putting a long leash on him, then letting him wander around until he is distracted and a short distance away from you. As soon as you hit the stim button, pull him gently towards you, and as soon as he makes a move in your direction, turn it off and tell him what a good boy he is. You can repeat this a few times until he begins to understand what the stim means and how he can turn it off. This method worked extremely well with my dog.