This question is very sad.
It is difficult to watch a loved one 'kill' themself with drug use, leaving you helpless. Unfortunately, there isn't a good answer for how you can stop them. Unfortunately, you cannot make a person stop. They have to want to stop using drugs.
I know this is not what you want to hear, but it is true. One way that you can make them want to stop is by expressing your own thoughts and feelings about your loved one's drug abuse. Have you had a heart-to-heart with the person? Have you told them how their drug abuse has affected your life?
This approach may work- making them realize how it is destroying, not only their own life, but the lives of everyone around them. They may not realize what they are doing to everyone else. They may think that they are only ruining their own life.