Michelle, you have a no smoking policy, and guests need to respect that. I'd reiterate it at the time of their booking via messaging.
However, what Kelly said is true. I am one of those dreaded smokers myself, but if I am staying somewhere where it is not allowed (almost everywhere these days), I will go out on the street to do so. But I don't toss my butts in the street- I will take them with me, and dispose of them in an appropriate place. It's possible that this was what your guest did. Put his butt in a can when he returned from his smoke (or carried the can out with him as an ashtray), not realizing that this is highly offensive to non-smokers. (I actually carry a sealed tin in my bag for butts, but most smokers don't)