Apparatus is simply hung on a deadbolt knob when the deadbolt is in locked position. It prevents the deadbolt knob from rotating enough to retract the deadbolt. This ensures that the door cannot be unlocked from the outside. A passage in an upper end of the apparatus is dimensioned to receive the deadbolt knob. The deadbolt knob is not round. The passage has side walls narrow enough to prevent the deadbolt knob from rotating freely within the passage. As the knob is rotated, the knob engages the side walls of the passage so that the entire apparatus rotates until the lower end of the apparatus engages the door frame. Many passages are provided so that the apparatus may be used with deadbolt knobs that have different locked positions.