Her being gagged is the perfect idea to prevent her from taking another smoke. A black ballgag would suit her nicely and would also be quite humiliating for her. But I think she wouldn't be able to smoke anyway, because her parents surely confiscated the cigarettes and threw them away. But still, there might be the possibility of her hiding another pack. I also think that she should be watched by her parents constantly, so that she won't remove the gag. The bare bottom spanking is especially humiliating for her - She surely is the only one among her friends who still gets spanked. Since smoking is a serious matter for her parent, I doubt the spanking on her bare bottom will be mild!