http://www.checkyourself.com/Stories.aspx
I don't know if you could stop them, only warn them, some of the clips above may make them take a little more notice. This is a great website I know its "American" but if one kid see's it and it makes them think twice then it doesn't matter what country they are in.
As part of my duties as a coach to the high school soccer team we discuss drugs once a month, mainly steroid use which is getting worse here in the U.S, but it allways ends up in discussions about who in the school is doing what, you will be amazed how much your kids know.