Chad,
Please don't put a starter to a running motor. You may have gotten away with it up until now, but you are an emergency room visit waiting to happen. That's not a very bright thing to do. The best way to stop a motor running backwards is to just shut the throttle down so the motor stops.

And yes, even spinning the motor the proper direction it can run backwards. It usually won't happen if you are using a starter motor, but will happen if you use a chicken stick to start your motor. As Chad said, the excess fuel in the cylinder causes the motor to kick back and run backwards.