A blocking device (2) for blocking access to an door unlocking device (118) of an elevator system (102) comprises: an outer element (4), which is introducible into an access opening (30) providing access to the door unlocking device (118); and an inner element (6), which is introducible into the outer element (4). The inner element (6) is configured for bringing the blocking device (2) into a fixing configuration, in which the outer element (4) is fixedly engaged with the access opening (30), by introducing the inner element (6) into the outer element (4).