What is the ruling on giving money to the needy if it is known that they will buy unlawful things with it, such as drugs and intoxicants, and is it obligatory to ensure that they do not use the money for unlawful purposes?

You can buy them what they need of food and clothes, or they can pay for permissible things themselves while you cover the bill, all while continuing to invite them [to Islam] and admonish them; for the rights of a neighbor are great, and the Prophet, peace and blessings be upon him, enjoined good treatment of neighbors.