Marijuana use is itself a symptom with a deeper cause. Like any other obsessive behavior it is often an escape mechanism for other stresses in life. Identify and deal with those stresses and you have a chance to deal with the substance abuse as well. Raise your children up with good Biblical morals, respect for parents and elders, and an open and forgiving attitude so they are never afraid to talk with you about their problems. And expose them to the Bible and its wisdom rather than using it as a bludgeon to show them how bad and sinful they are. Leave room for the Holy Spirit to do his work, and pray for your kids and those of others. Even then, some kids will not listen. That will be on themselves.