With it not breaking the ToS is really the point as people suggesting if these were push to shout chat, it's more visible therefore more people report them, but they're not going to go after anybody if it turns out they've not broken the ToS and those ads that minors might see not only stay up, but are more visible to them. With regards to the adult website comparison, the law may not necessarily see it that way as both parties would be engaging in sexual activity, whether it's erotic writing or cyber sex, if it involves a minor then it's the adult that gets into trouble. Somewhere like here in the UK that could potentially ruin your life because you'd be tried as a sex offender which means you're on a sex offenders list and that restricts you in so many ways. And it doesn't matter if you thought it was an adult, some laws on this can be pretty brutal. So I view it as a risk, sure there's things people can do to mitigate that risk and police their communities to make it a slim chance of happening to them.
From a moderation perspective, I don't see any of these suggestions (including my own) to really be any real solution...including the scorched earth one. All scorched earth does is create problems for people who're completely innocent of wrong doing and in effect punish them for another's actions whilst just pushing these communities underground and in the shadows. It means people end up doing this in secret, which sounds like it'd keep underaged people safe, because there's less in the way of advertisement for minors to see. But there's also less in the way of people co-operating with the game's moderation and a disincentive to report suspicious behaviour from people within those communities because hey, they don't want to get banned. And the visibility for GM's goes waaaay down.
And this is where I draw back to a previous point. YoshiP said they have legal support in most countries for investigating these. I expect their strategy is based on what data they have and what legal advice they have. I strongly suspect they already have their best strategy for dealing with these. YoshiP's advice was to report them and they can investigate. If people keep doing that when they see things like brothels being publically advertised then the best thing you can do is report them.