You shouldn't have to live with the smell of someone else's habit. And you, your children, and your things shouldn't have to carry the smell . Talk to the neighbor one more time and then call the landlord every time. Threaten to break your lease if he (or she, whoever owns the apartments) doesn't enforce the no smoking rules.
That said, if you live in an apartment, people smoke. That's pretty normal. Does it suck? Yes. Is there really anything you can do to stop people from smoking (anything) in their apartment? Not really. I'd have more of a problem with my neighbors smoking cigarettes (because they tend to smoke a lot more of them and the smell is awful, in my opinion) and the smell seeping through. Also, cigarettes tend to leave a stink behind that weed doesn't.
And I'm totally lol'ing at the posters who think you should call the police, do not pass go. Really now? I use marijuana (even during pregnancy! *gasp* to keep my blood pressure low and help me sleep. So I'm trying to think what I would do if my neighbors came to me and said they could smell it and could I stop. I would do it outside instead. But realistically, if you asked they won't alter their habits, I would tell the landlord you will break the lease and see them in court if they don't stop it. If it's a smoking apartment, however, I would just deal with it, put up my own fans, and be glad my neighbors weren't chain smoking Marlboros next door. That stuff stinks!
As a side note...
"Then I would ask to see the renter's medical marijuana license. If he's legitimate, then work with him to reduce his impact on others who do not wish to breathe in his mind-altering fumes.
If he does not have a legal permit, I don't see how it is ethical to not turn him in for committing a crime. While I am sure some pot-heads are nice people too, if something's illegal, it's illegal. It's really very black & white.
Would it be any different if the person were smoking crack cocaine or heroin? They're all illegal."
Um... wow! Do you always let the government make your ethical decisions or do you ever stop to consider the issues for yourself? It's hardly black and white when it comes to law. It's pretty unfair to compare a marijuana smoker to crack head, don't you think? Clearly you've never smoked marijuana before!