Smoking is a very unhealthy habit, not just for yourself but everyone around you who is exposed. The chemicals linger in paint and upholstery as well, further exposing everyone who comes in contact for years to come.
ANYTHING you can do toward quitting smoking is a good thing. Chastity devices are certainly worth a try. Keep in mind you will suffer doubly doing it this way, but blue balls mght be the distraction you need to help you quit. Make sure the device you use fits properly and is totally escape-proof. It may take several purchases before you find the right one, but inexpensive cages of various styles can be found on eBay. Your girlfriend will need to be iron-willed and sadistic enough to not unlock you no matter how much you beg, and the keys must be well hidden. Good luck with it.