How is it taunting if she doesn't want to say it back? The entirety of her response could be, "Yep." It wouldn't be taunting for someone to tell me, "You can't do nuclear physics." I would agree with them and be slightly confused why they were apparently out of the blue stating it.

If she's truly being randomly bullied, that's not going to be solved by telling black people they can't use that word. A bully would just say something else. This is a rather easy one to deflect.