You'd think that repeatedly telling someone not to drink or smoke would be enough. But your brain can do very funny things. Your brain—specifically the part of the brain that influences cravings, your insula— hears "don't smoke" and reacts as if it hears "smoke." And that stimulates the craving for smoking.
A much better approach when you want to help someone get rid of an addiction is to flip the message. Instead of saying "Don't smoke," say, "Breathe free." Instead of "Don't eat doughnuts," say, "Have a handful of nuts."