I caught my teenager smoking. how can i effectively stop this behavior?

Good luck: Every teen experiments. Depending on how much control you have over junior, you can "lay down the law" and this would be a kindness in the long run. Most effective is your being a non-smoker, not tolerating it in the home (that one you can do), and especially reminding junior that he/she is making some cynical jackass tobacco company ceo rich.