Seriously. I've had that problem since I became a "Dr." and "Prof.," no matter how many times and how many ways I tell them. I don't even really object to "Ms," but "Mrs. Lastname" not only irks me, but it's inaccurate. Yes, I'm married, but I didn't change my last name. One semester, I had a student who just called me "Mrs." Not "Mrs. Name." Just "Mrs." Not only did he ignore my weekly corrections, he never bothered to learn my name. I finally just kicked him out of class one day. At that point, he just stopped addressing me as anything.