What is the punishment for a sister if she marries a Kitabi man despite attempts to dissuade her, and is it permissible to intervene to prevent her, even if the father objects?

The scholars of the Ummah have unanimously agreed on the prohibition of a Muslim woman marrying a non-Muslim. If she marries him, her marriage is null and void, and her cohabitation with him is considered zina (fornication/adultery). She must be advised gently and prevented from this, as her father is responsible for her. If he fails to prevent her, then the questioner must prevent her and ostracize her if ostracism is likely to benefit her, and seek a Muslim husband for her.