A mechanical firestop system is designed to mechanically prevent the spread of fire through openings in a structure. Among the given options, a firestop block is specifically designed to serve this purpose by blocking penetrations in walls and floors, thus effectively stopping the passage of fire and smoke without the need for additional chemicals or treatments. Cementitious systems and intumescent sheets may also prevent fire spread, but they typically do so through chemical reactions rather than mechanical means. Cable transits are not typically considered mechanical firestop systems; they are used for routing cables while maintaining fire barrier integrity.