I am having a similar problem with the guy downstairs chain smoking on his balcony. After reading through all the responses (90% useless and coming from idiot trolls), I think I will install a smoke detector, contact the Mietverein, and ask a lawyer. That is if talking to the guy doesn't help (he is actually schizophrenic). He doesn't seem to get the hint when I slam the door when he is smoking..
I smelled cigarettes on my clothes at work the last week, so it is getting out of hand. The apartment smells like stale smoke. For you captain Obvious' out there, I do shut the door.
This post is more of a plan than a question, so I will update it when I get more information.