A device includes a base secured in a car, a stopping block displaceable on the base, an actuating member for displacing the stopping block with, and a lock block secured around a steel rope of a control of a hood of the car, which control can be used to pull the steel rope for making the hood become openable; the stopping block has a big hole, and an horizontal slot communicating with the big hole and as high as center of the hole; the rope is passed through the stopping block with the lock block arranged behind the stopping block; the lock block can't pass through the slot; the stopping block is displaceable to such position as to exactly face the lock block at the slot, preventing the rope from being displaced; thus, thieves can't open the hood to cut off the power to the alarm for stopping sounding of the alarm.