That's the danger.. The more you say they ain't allowed to do something that all their mates are doing the more it appeals to them and the more determined they become to do it behind your back IMO. Once they've done that, if anything untoward was to kick off on it the harder it's going to be to bring it to you.. That's the way I look at it, it's hard to find a balance..
I think saying "well I wish you well, NOW OUT!!!"....should do the trick.