Well, before I became an holistic therapist, I originally trained as a chef and also a baker.
When I was doing my work experience in a bakery, I was told that making bread, as in shaping the bread once its been mixed by a machine (as NO bakery makes it from scratch), removed nicotine stains.
After a shift smokers fingers would be as clean as non-smokers!!!!
So, has that put you off bread? think about all the bakers in your local bakery, think about picking up your fresh bread in the supermarket, how many of the workers smoke? Have they been handling the bread dough?
I have NEVER smoked, and it repulses me. And I must admit, if I drive past a restaurant or pub and I see the kitchen staff outside puffing away, I rarely give them my custom.
So, if you're customers want to get rid, tell them to make up a batch of bread. The packet stuff is fine, as long as they are kneading it and shaping it.