May I turn down a prospective spouse because they smoke?

Yes. You are entitled to choose a spouse on the basis of habits that affect health, finances and the family, and smoking has been ruled forbidden by the National Fatwa Committee Muzakarah.
The Prophet ﷺ made faith the basis for choosing a spouse, and told families to accept a suitor whose faith and character they are pleased with (Jamiʿ at-Tirmidhi 1084); habits that harm oneself and others (Sunan Ibn Majah 2340) may rightly be taken into account. If you are still interested, discuss it honestly: are they willing to quit, with a clinic's help, before marriage? Promises to quit "after the wedding" without effort now are rarely kept. Turn them down kindly without insult. See how to turn down a proposal kindly.