You're absolutely right buddy. Believe me though it's not as hard as you think it is. You'd be surprised how effective words can be when you use the right ones. Just straight out ask them "why don't you ever buy any weed and smoke us on it?". Regardless of their answer, just break it to them that weed does not grow on tress (it gows in little bushes!!), and that you and your friends simply cannot smoke them on your weed all the time. Tell them to come back when they start bringing their own stash. And next time someone says that Michelle wants to smoke a bowl, just yell out "have her buy her own fucking pot!!!".