You let them continue until they can get a spot in a medical setting where they can safely withdraw.
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that safe setting may very well be the shelter we are talking about
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you are steering away from subject. it is absolutely fair to tell them “being homeless and nuisance in the street is from now on illegal. either you want help to get back to society and then you will accept the help with its terms - you are really not in a position to make demands - or you can move to some unabomber cabin in the middle of nowhere, and there you can do whatever you want”