code: def unregister_observer(observer_class, observer_id)
        @observer_mutex.synchronize do
          log "EventListener: un-registering observer #{observer_class} #{observer_id}"
          if (consumer = @observers.delete(observer_key_for(observer_class, observer_id)))
            consumer.cancel
          end
        end
      end
docs: Unregister an observer instance, and cancel consumption of messages. Any pre-fetched messages will be returned to the queue.