Blocking device for a pump having a rotor that is rotatable about a rotation axis in a pump duct and comprises a rotor hub and a rotor collar that extends from the rotor hub in the radial direction and encircles it in an undulating manner, characterized in that the blocking device comprises a plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) which are configured to block the pump duct in the axial direction on both sides of the rotor collar, wherein each of the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) has a slot with a U-shaped sealing profile for abutting against the rotor collar, a sealing face for abutting against the rotor hub, and two contacting faces for abutting against a seat (60, 66) of the pump duct and/or against a contacting face (62, 66) of another blocking element (52, 52a, 52b, 52c, 52d, 52e) of the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e), wherein the blocking device has a first seat for the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) on the side of the first inlet/outlet space, against which a first one of the blocking elements (52, 52a, 52b, 52c, 52d, 52e) abuts by way of a first end contacting face in a first operating direction for pumping from the first inlet/outlet space to the second inlet/outlet space, and has a second seat for the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) on the side of the second inlet/outlet space, against which a last one of the blocking elements (52, 52a, 52b, 52c, 52d, 52e) abuts by way of a second end contacting face in a second operating direction for pumping from the second inlet/outlet space to the first inlet/outlet space, and the spacing between the first seat and the second seat in the circumferential direction is greater than the spacing between the respective first and second end contacting faces (62, 66) of the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) in the circumferential direction. characterized in that the blocking device comprises a plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) which are configured to block the pump duct in the axial direction on both sides of the rotor collar, wherein each of the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) has a slot with a U-shaped sealing profile for abutting against the rotor collar, a sealing face for abutting against the rotor hub, and two contacting faces for abutting against a seat (60, 66) of the pump duct and/or against a contacting face (62, 66) of another blocking element (52, 52a, 52b, 52c, 52d, 52e) of the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e), wherein the blocking device has a first seat for the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) on the side of the first inlet/outlet space, against which a first one of the blocking elements (52, 52a, 52b, 52c, 52d, 52e) abuts by way of a first end contacting face in a first operating direction for pumping from the first inlet/outlet space to the second inlet/outlet space, and has a second seat for the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) on the side of the second inlet/outlet space, against which a last one of the blocking elements (52, 52a, 52b, 52c, 52d, 52e) abuts by way of a second end contacting face in a second operating direction for pumping from the second inlet/outlet space to the first inlet/outlet space, and the spacing between the first seat and the second seat in the circumferential direction is greater than the spacing between the respective first and second end contacting faces (62, 66) of the plurality of blocking elements (52, 52a, 52b, 52c, 52d, 52e) in the circumferential direction.