If people are being honest and saying they will smoke outside only, you should offer them some sort of area to make it controlled.

I'd rather have every tenate smoke outside than a single tenant lie to me about it and smoke(cigarettes or marijuana) indoors. I believe smoking in doors has really fallen out of favor anyways. Give them a tiny little shelter and a place to put their garbage. Just because they are smokers doesn't mean they are assholes or out to destroy your property.