Most people open up the motors and remove the breaks. You can just apply power to the break line to turn them off (I would recommend a tip transistor to a relay rather than wasting an Hbridge for a simple on/off) but it will use up a lot of battery power just to keep the breaks open. On the other hand, makes a nice failsafe for a device with enough power to hurt you and even kill a pet or child that gets in the way.

Alan