What is the ruling on sending reminder messages about adhkar (remembrances of Allah) and the day of Ashura to a specific person after having made a covenant with Allah to repent and sever the relationship due to the impossibility of marriage?

Repentance is completed by severing the relationship entirely and not sending messages between you, even if they are reminders or admonitions, to avoid recalling the past and continuing the affection that could lead to a return to what you were doing. Therefore, ask him not to send any more messages, and if he insists, ignore him and do not reply to him.