You let them leach - take a stand, kick them out, lock up your stuff and refuse to give any more.
listen your intuition before helping others; sometimes you know when to help-people and when not to, don't get mad at the person you should have not fooled with them in the fist place, just ignore them.
It doesn't matter, whether they are drug addicts or whatever. Tell them NO.
Kick them all out. It isn't right that they are forcing you to support their drug habits. Let them know that the reason you have what you have is because you don't have a drug habit and you are sick and tired of being dragged down by theirs. It's called tough love and they won't like it. But the way things are now are not good for any of you.
...because they don't know how to think otherwise. goes to sow you never can tell..c'est la vie