The invention relates to a shut-off device comprising a plug intended to be placed over a threaded orifice of a casing, or to be removed therefrom, by screwing and unscrewing respectively, the plug comprising a central body bearing a screw thread, and locking and unlocking means that oppose the spontaneous unscrewing of the plug and that are activated automatically by the turning of the plug and its corresponding progression in the direction of screwing along a longitudinal axis. The locking and unlocking means comprise two releaseable coupling members, of which a first member is coupled to and turns with the central body via a protective skirt and of which a second member can be prevented from turning with respect to the casing during screwing.