A blocking device, and a gear-drive unit containing such a blocking device, and a method for producing such a gear-drive unit for blocking a rotary motion of a shaft (14) relative to a housing (16) of the gear-drive unit (10), having a first, rotatable blocking element (32) and a second blocking element (34) which is displaceable relative to the first blocking element (32) by means of at least one electromagnet (44) and at least one restoring element (42), in which the blocking elements (32, 34) in the blocked state mesh in the axial direction with one another by form-locking, and the blocking device (30) is embodied as a separate, independent structural unit (31), which can be mounted as a unit (31) on the one hand onto the housing (16) and on the other onto the shaft (14).