Is it obligatory to refuse marriage to a specific person if a covenant was previously made with God not to think about them again?

If you said, "I covenant with Allah concerning such-and-such," and then broke your covenant, you must offer expiation for an oath. Your intention is considered here. So, if you intended not to accept him as a suitor and then accepted him, you have broken your covenant and must offer expiation. However, if your intention did not include that meaning, you do not break your covenant by accepting him. It is preferable to avoid the صيغة (formulation) of covenanting with Allah, because the matter of a covenant is grave.