What is the ruling on smoking, and what is the ruling on preventing a smoker from leading the prayer?

Smoking is forbidden. It is not permissible to appoint a smoker as an Imam for prayer because persistence in smoking is considered فسق (disobedience/transgression). Whoever strives to prevent a smoker from leading the prayer has done well. However, if a smoker does lead the prayer, his prayer and the prayer of those behind him are valid. It is obligatory to advise him and warn him of the harms of smoking.