After some thinking it over and some talking with other teachers, I decided to ask the ratings site to delete the narrative portion of the comment only. The angry student had every right to express his or her negative opinion of my teaching; I think that's the primary purpose of those kind of sites, really. On the other hand, anonymously calling me a bitch and a racist on a published site was defamatory and libelous. I have every right to protect my teaching reputation against ungrounded assault. I guess that makes me a race-supremacist silencer, someone who uses my class and race privilege
to manipulate the "truth" and "white-out" discomforting critiques. Yeah, I read that post-colonial theory text too.
I am happy to report that the site was happy to comply. Now we've all gotten something of what we wanted, maybe the best we could do under the circumstances. The student has gotten to rat out the professor, the professor has been alerted to an intense negative reaction, and no lasting defamation of character is going to linger.