It's a tricky situation. Can really emphasise as I had a similiar problem. In our case it was three young women in the flat below - all of them and their friends smoking inside with a lot of smoke and smells in our flat above them. We had a big shared garden but they couldn't be bothered to go outside. In the end what worked was that, after talking with them etc, I kept contacting my neighbour who was renting his flat to them until he came round to investigate. He was horrified by the smell, colour of walls etc, also it was in our joint lease that smoking wasn't allowed, and gave them notice. It was really wearing and stressful but worked in the end.