Question: I am a beginning graduate student in Mathematics. I have a professor who always brings up the fact that my GPA is not great in my undergraduate degree even though my performance is above the norm in the institution I am enrolled at the moment, so far I got straight A's in the graduate courses that I have completed. He always tries to point out at my flaws in my undergraduate years to humiliate me in front of other professors and my colleagues. What would be a wise reply/reaction to such abuse? 

I feel bad about it cause I feel like I am harassed and I don't reply.
I am also currently taking a course with that professor.

Answer: 
I've gone through this nonsense before. Here is an interaction I had with one of my professors regarding an assignment I had in college, around 20 years ago:

> 
> Me: Sir, I know what pride is but what is prejudice?  
> 
>  Him: Prejudice means, "what I have against you"
> 
> 
> 

Best part is, I still did not understand.

That professor is looking for a reaction from you. Detach from the negative experience, don't react negatively and you will stop the very thing that feeds their ego.

You have to attack the core of his belief system.
It would appear that he is trying to shame you to make himself feel better. This is not unusual behaviour, though you would hope for better from a senior academic. One solution is to call him on it enough to stop it but not enough to make an enemy of him: 

"Yes, my GPA was indeed 3.0. That seems to have really caught your attention because you've mentioned it the last 9 times we've met."

I would not try to emphasis that you are getting great grades now. The important bit is to call him on his GPA-focus, particularly if there are other people around. Do it in a manner which suggests that you are genuinely curious about why he is mentioning it, rather than letting him know that it irks you.