Its easy tell them they can smoke as much as they like so long as they have the dv window open!. In all seriousness do you think the passengers cant smell smoke from the flightdeck?, but you tell them that they cannot!.
There are so many reasons not to smoke in aircraft full stop apart from being a fire risk and second hand smoke, the aircraft really stinks forever the smell never goes away. Smoking bans in the workplace are becoming more and more common anyway so soon it will be law if it is not already.