How well do you know them? Do you ever see who it is that is smoking? I know when I was a teenager, I would smoke in the back yard and throw them over the fence to hide the evidence. How many butts end up in your yard?
I am willing to bet its a teenager disposing of the evidence... or a really inconsiderate human being (which is quite possibly either or)
If its some prick, I would just take a crap in their front yard. And then blame it on a really big dog if he comes a knockin.....