A protective device comprising a covering element associated slidingly with a plate which can be fixed to the door or leaf and can be positioned selectively so as to close an opening for access to the lock which is provided in the plate. Two pins associated transversely with the plate block temporarily sliding of the covering element. The pins interact with the arms of a rotor, associated rotatably, within a cavity provided in the plate. Pivots protrude from the rotor and cooperate selectively with at least three cams, freely associated rotatably proximate to a surface of the cavity. The cams have an axis at the vertices of a polygon and interact with a magnet-based combination element adapted to modify the angular position of the cams to allow or prevent the rotation of the rotor and temporary mutual release of the pins and the covering element.