Just what can be done to ensure limited exposure of this kind?
There isn't much you can do sadly. A teenager is smart enough to keep his traces secret and dumb enough to think he'll get rich gambling. Talking to him is the only option in a situation like this. If he somehow gets what you are saying, he will never ever become addicted to gambling again. If he doesn't, then you are looking at another poor lost soul who's going to waste his all remaining life on gambling. That's the problem with some people. They hear the words but their brain can't comprehend their meaning. Smart people know their limits, they instantly recognize it when they see a person that is smarter than them. Dumb people think they are running the show.
I kinda agree with you but do you know that all teenagers are not the same? Some can be smart enough to cover their tracks and Make sure that no one knows that they are into such activity, but it doesn't matter, like you said there's a limit to what you can do. Talking to them and educating is the only solution. At that stage of life majority of them wouldn't listen because of they might be exhibiting some sort of youthful exhubirance. This might be out of context but as adults we must also show good example, the excuse these teenagers have is that they see their elders doing it so they just emulate them.
There would really be tons of things that could be able to influence those young people specially if it's something that caught or catch their interest and this is why as a parent or guardian then it will really be that recommended that having that proper guidance and educating them will really be that something that needed. Yes, there might be those who don't listen up but its always better that you should do something as a parent. Do your responsibility at least and it's up to them if they would really be going against you.