now if you want your servos active, just not the motor, put the switch on one of the motor wires and that will keep your servos free to move, just not the motor
If this is a brushless application, you just cannot isolate the motor by adding a switch to only one lead to the motor. Since the system is a three-phase "pulsed DC" application, you might do damage by opening up one segment to the motor.
CGr.