What is the ruling on giving charity to someone who will buy cigarettes with it, knowing that he might ask for money to buy food, then use it to buy cigarettes?

It is not permissible for the one giving charity to give their alms to a person when they know, or have strong reason to believe, that the recipient will use it for something forbidden. This is based on the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." If the recipient is mentally unstable, then money should not be given to them at all, based on the Almighty's saying: "And do not give the incompetent your wealth, which Allah has made a means of support for you."