What is the ruling regarding a man telling his future wife, "You are forbidden to me" if she does something he dislikes, and then she does that very thing after marriage without him having forbidden her? And is she Islamically forbidden to him?

Such words spoken before the marriage contract have no effect on prohibiting the woman after marriage, whether it was a promise of prohibition, a suspension of divorce, or dhahār, which is not valid before the contract according to the majority of scholars. One should repent from establishing a relationship between a man and a woman before marriage.