We did that with Prohibition. It was a disaster that ushered in organized crime rackets into every major city in the USA and probably caused more problems than it ever hoped to solve.
Libertarians are simply wrong on issues of drugs being legalized. Actually, they are off base on many of their positions. They seem to ignore God and that is their downfall philosophically.
The problem is that drugs' currently being illegal is not stopping the people who want to use them from using them. They find ways to get around the law. Drugs have been illegal in the USA since the late 1930s and even with the heavy-handed approach that many law enforcement agencies take with druggies today, overall drug use is worse than it ever has been.
In every society, there will exist a group of people who will refuse to control their own passions and engage in self-destructive behavior. This simply cannot be helped unless the person involved wants to be helped. Taking the "banning it" approach to these problems will simply move the problem to the next form of moral malaise that is still legal.
As for ignoring God, I am going to assume that you are speaking about the Christian God. If this is not the case, I apologize. But there are no teachings in the New Testament which task the state with enforcing the Law of God. Furthermore, we are taught as Christians that we must obey all of the laws except for those which conflict with God's Law. Even if drugs are legalized, the people who take their faith seriously will not indulge in them to excess, or at all.
See #14...
Prohibition was constitutionally implemented, by the way...
People might not like it, but it was done fair and square... It was also repealed constitutionally...
The Constitution DOES work.