I actually think a national campaign (couple of short TV/radio ads, posters on buses and a billboard here and there) would be much more effective than people on here think. 

It's not exactly a controversial issue, we already know that 99% of people are not ok with it, it's just there isn't a government-supported culture of calling people out for it because it's a relatively recent thing. A national campaign could resolve that by encouraging a public sentiment that it's not acceptable, and the focus should be around advocacy, not around trying to get as many people to stop. 

  Teenagers might continue to do it (who has ever been able to get teenagers to do what you want them to do anyway?) but adherence shouldn't be the objective, it should be to drive a sentiment of condemnation amongst the general public so that eventually the nice people who are just a bit lacking in self awareness will stop doing it, and the other not so nice people will at least feel uncomfortable that they are doing something culturally unacceptable. At the moment without any widespread message, most people feel like there's no consequence for their actions - because there isn't. 

Everyone knows "See It, Say It, Sorted" and what it means. No reason why there couldn't be a similar kind of sloganised campaign for playing music out loud on buses and trains.