Q: Is Marijuana Haram ?
Q: Haram, according to every marjaa'
Q: Why ?
A: Marijuana is an intoxicant. As per famous hadith from Prophet Muhammad(p.b.u.h) accepted by all muslims 'Any substance that intoxicates is 'khimr'(wine). This means that all intoxicants are equivalent to wine in their prohibition, as wine is explicitly made haram in the Holy Quran.
Stopping someone from it depends on the level addiction.
Discussing with the person and explaining the danger of it is the first step. The person may need rehabilitation if the addiction is of high level.