> You can stop a junkie from shooting up by imprisoning them, or better yet, putting them through drug rehab.
Those responses have been tried for many years, with very poor results. Also, it's not clear that there is anything criminal about the vice of drugs, and thus taking away their freedom would be highly unjust. We take away people's freedom for bad behavior or because other people are uncomfortable.
Based on the little I know, what does help people is giving them access to social services. I think several American cities are pioneering bringing the services to the person - when authorities are called, they don't send police or only police, but people who can connect the person to various agencies.
Apparently this approach results in a much higher rate of service use (which makes sense commercially - if someone had to wait in long lines and fill out dozens of forms, and then face long delays for service, you wouldn't have many customers. If someone showed up, when need the service, and offered it on the spot, you'd sell a few more). And those people gain more stable lives and are able to better care for themselves.
We can see plenty of wealthy professionals turn to damaging behavior under great stress. Imagine how they would do if added to that stress is having no roof or reliable food, the loss of safety, the loss of any credibility - even enough to get basic jobs, the harassment, etc. But then them: 'We'll take care of the shelter, food, etc., and here are classes to help cope with your other problems without using drugs', and I can imagine they would have a much better chance.