(Not a teacher)
Generally, people use surname and address when writing an email. There is nothing wrong in it. If you are so specific about the first name, it can modified as;
Thank you for the email.
Please note that I had already informed NOT to address by surname and the same was agreed by you. But you have failed to keep up your promise, hence, bringing to your notice again. "Please ensure in future to call me by first name instead of surname."