The harm principle is a prohibition on banning things that don't cause any harm to others. It is not a sufficient justification for banning anything. Some bans are worse than the harms they would prevent. And many bans are wrong because they are ineffective at preventing the harms they are intended to prevent.
The problem is that it can be difficult to be seen to agree completely that a cultural meme like body fascism is a big problem causing considerable harm to many people, while at the same time rejecting a particular social engineering solution to it. Speaker after speaker spoke of the harm done by unrealistic expectations of plastic beauty, as if this were the point of dispute.
Sure, there are those - the libertarians - who would interpret harm in the narrowest possible way - the initiation of force - in order to delegitimise almost every law. At the other extreme communists might argue that competition causes harm - which it does to those who lose - and reject economic liberalism. Both may claim, wrongly, to be good Millians.
The error both are making is in looking for a rules-based formula for when something ought to be banned or not, and then arguing over the interpretation of the rules. And if you misunderstand the harm principle as saying what must be banned, rather than what must not be banned, it is understandable that you might want to adopt a narrow concept of harm, to minimise the assault on liberty.
I suggest that the harm principle should be seen in the context of Mill's utilitarianism. Specifically, that goods or ills have to be weighed up against each other. Some precious liberty against some moderate harm? It's a judgement call, and the answer is often a boring compromise: 70mph; no parking between 9am and 6pm; hotels must have fire alarms...
And the most useful liberties - such as the freedom to compete in business - are thus justified in spite of the legions of bankrupted suicidal failures.
So the communist has a broad concept of harm and prohibits a great deal. The libertarian has a narrow concept of harm and prohibits little. I agree with the broad concept of harm, but I would still prohibit only a little, understanding that the harm principle does not demand a prohibition.
So back to the airbrush ban. I suggest that the correct and healthy attitude to have towards advertising, celebrity culture and so on is a skeptical one. What you are seeing is not real. The danger is that an airbrush ban might make you think it is real, compounding the negative impact of the image. I don't see a prohibition intended to increase the confidence you might have in the fidelity of body fascistic images in the media as worth any loss of liberty.
Meanwhile I am looking forward to the men's policy paper. I continually find my self-esteem undermined by the portrayal in the media of men who are richer and more powerful than I am. I worry about all the boys facing these impossible comparisons. Something must be done.