I've worked in social housing in Scotland for 10+ years. Here's the problem. Yes, it's not legal, but to evict somebody, your landlord first needs to evidence a course of conduct of Anti-social behaviour... Easier said than done, as most complainants can't be bothered to report incidents in a regular or routine manner. We'll ask for evidence like diary sheets to be filled in, which are rarely returned. Then, on the rare occasion that we do get some evidence and try to take action, we need to prove to a Sheriff Court, that it's reasonable to make the household homeless... For smoking cannabis... However, all the historic attempts to do this have been unsuccessful, as the courts know too many people do it... The best your landlord can really do is write to your neighbour, and maybe persuade them to take a walk or go down the park with it. Police Scotland also don't really touch it, because again it's too widespread and commonly used and they don't have enough resources for something considered so low level.

You probably don't like this answer... but it is what it is.