As I read your replies, I realize what a difficult position we are in. I agree about non-smoking policies. I know people are confused about medical marijuana. However, my biggest concern is that we have been placed in the position of being the police, the judge, the juror, the enforcer...and sadly that's what many of your replies sound like.
Please soften your approach...put out occasional reminders to all residents that it is nonsmoking and say it ways that don't sound like Hitler. The rock and hard place is that they are paying for a 'home' and a certain amount of privacy. No, they can't smoke in nonsmoking buildings/properties. And yes, there will be ramifications. Just work on being polite, kind and customer service oriented. We can say 'no' in less offensive ways. Happy New Year!