The blocking mechanism is formed by a wrap-spring coupling which has a stationary cylinder part (2c) and a cylinder part (bush) (2b) rotatable about the cylinder axis (2a) as well as a wrap spring (1) which is fastened at one end (1b) to the stationary cylinder part (2c). A blocking lever (4) is fastened to the rotatable cylinder part (2b), the centre of rotation of the blocking lever (4) being the cylinder axis (2a). A magnetic clamp (11) is arranged on the blocking lever (4) and forms a magnetic adhesive connection with the driving plate (5) which is arranged mounted rotatably on the rotatable cylinder part (2b) and which is pulled up against a stop (8) by means of a tension spring (7). The driving plate (5) has a stop (6) for the free end (1a) of the wrap spring (1). <IMAGE>
