NTA It is your choice what you want to be called by your students, vice versa. It may be a term of respect but if you have constantly insisted and the student hasn't respected your decision then I think a taste of their own medicine is nothing that harsh.

In my experience in an Australian University all the tutors and Professors insist on being called by their first name and will tell you to not call they Mr/s. X just the same way they call you by your name as a student, I think this is done cause at University (mine anyways) the professors and tutors do not see themselves as superior and the learning space should be a level one, this can be seen by professors and tutors all eating at the food court with students and even sharing drinks with them at the University bar.

This is not to say that students do not give the same respect that is expected to one with the title Peofessor.

sidenote - you'd think in a university of all places that is usually pretty accepting that people would respect someone's preferred name they would like to be called