Stops the countdown

def stop(label, at=None, remove_from_labels=False, stop_once=True):
    """Stops the countdown"""
    t = at if at is not None else time.time()

    if label not in labels:
        return None

    timer = Marker().loads(labels[label])

    if timer.is_running() or (timer.is_stopped() and not stop_once):
        timer.stop(t)

    if remove_from_labels:
        del labels[label]
    else:
        labels[label] = timer.dumps()

    return timer.duration()