The rear brake is off... do you mean that the rear brake has been physically removed from the bike? The motor doesn't stop... does the motor not stop (as in it can't be turned off), or does the bike not stop because of issues with the brake? I'm going out on the limb, but if the rear brake is missing, I would think you have a problem there. If the motor doesn't stop (cannot be turned off) either through the use of the key or the kill switch, the problem isn't related to the rear brake, missing or not.