Alcohol and drugs are banned in prison. People still seem to get both in there. Any hope of preventing their use through the use of government force is a futile effort. The best method is educating people.
I think people talk to their teens in the completely wrong way about drugs and alcohol. You can't just tell them not to do them or that you don't want them to do them, etc. As teenagers, they are trying to find their level of independence and trying to grow that independence. Telling them that they can't do drugs will just create a rebellious feeling about the issue. When I talked to my kids about this subject, I told them that they are free people and are free to make their own decisions. However, they need to figure out for themselves if this is something they need in their lives. Tell them that you are hopeful that they will decide on their own that they don't need them.
I always laugh at those who think they can legislate a sense of morality. Prohibition was already attempted once and all that gave us was the Chicago mobsters and gangsters who brought it in from Canada, much as the current gangs bring drugs in from Mexico and other points south. People have the right to make bad decisions. If a man drives drunk and kills someone, you charge him with the crime of murder. It's as simple as that.