@Moira21 So "enforcement" in this scenario (which really doesn't sound unlikely to me) equals a stern talking-to, after which you both leave each other negative reviews.
And that's the kicker. Any time you call a guest out on breaking a rule, they're going to knock you down on your ratings. That doesn't mean hosts shouldn't stand behind their rules, but it does make it imperative that the rules are clear, unambiguous, and well worth the consequences.
Also, they're called HOUSE Rules for a reason; your right to police guests' conduct does not extend beyond your property. And besides, "non-smoker" is not a fixed or static character trait; it's entirely plausible that a guest who doesn't usually smoke will decide to do so while they're visiting Blackpool, just as a self-described "vegetarian" might decide to have a hot dog or a self-described "straight guy" might decide to have a little man-on-man action, and it's absolutely none of their Airbnb host's business.
We can agree to disagree on this, but for other hosts seeking advice here I would strongly encourage limiting your house rules to guests' willful conduct on your property.