Leave them, or threaten to leave them. Let them know that if they don't stop smoking in a month and a half, you're gone. You'll no longer support their habit with your presence.
Does your father smoke tobacco?
Let him know you don't want to watch him die a slow agonizing death. Take him to a house somewhere where the Hospice nurses are there with the family everyday, the family of a man who smoked marlboros more than he drank Water. Let your father witness, second hand, the sunken in face, the discomfort of constipation from all the pain meds, the anxiety, the challenges of walking to the next room over. Tell him to look into the eyes if those loved ones who know he's leaving the planet too soon. My father left the planet too soon.
Does your neighbor smoke?
Ask them not to smoke upwind from your house. Let them know you don't like the smell or the non-health benefits. Make them have to think twice about their addiction when they step outside to get unfresh air.
Does your niece smoke camels?
Tell her how bad it is for her skin, how it will give her wrinkles earlier. Tell her it will stain her teeth too and give her a not-so-pretty smile. She's probably really into her image right now, maybe it will help her quit. Inform her that the tobacco industry targets young people, in their sneaky little ways, to make them think smoking is cool, in hopes to get them hooked for life.
Do you smoke chemical-laden tobacco on a daily basis? Do you know why you smoke? Do you know why you wouldn't stop smoking?
You can go ahead and up your chances for the most common diseases like cancer and heart disease, we've all gotta go somehow... Just want to let you know for your family's sake, that watching a loved one decay like a Holocaust victim while tobacco companies' pockets grow deeper, is nothing short of morbid.