A locking-stressing-mechanism with spring-actuated output drive and a counter with which an apparatus of this kind is fitted, accommodated in a two part housing the two parts of which are mounted to be rotatable relative to each other, can be blocked by means of a pre-stressed leaf spring. The leaf spring is initially accommodated in a recess in the wall of one housing part. As soon as the permitted number of actuations has been reached a push rod pushes the leaf spring out of its resting position. The leaf spring then jumps into a recess in the wall of the other housing part and the two housing parts can no longer be rotated relative to each other. The push rod may be mounted on the pointer of the counter. This blocking device can only be overcome by the application of a force which is sufficient to destroy the device. The device is suitable for blocking a high pressure atomiser or a needleless injector with which a fluid is atomised to form an aerosol or a fluid is injected into a biological tissue.