Having dealt with this before I can tell you that least explosive option is try to create a safe, designated are for them to do whatever they're going to do. You can say, "do whatever you're doing in this shed/garage/whereever", and they'll probably keep it mostly to that area, or you can say dont bring it anywhere near my property, and you'll be calling a plumber a month from now because your toilet is clogged with needle caps or whatever bullshit they do to try to hide it.
You know how addicts behave by now. If they're doing hard drugs again giving them an ultimatum isnt going to make them stop, its just going to make them have another explosive blowout and make you feel like shit when they start screaming about how you ruined their life because you wont let them smoke crack in your apartment.