That said as much as I think that that professor is a door knob that should’ve been fired instead of promoted I also don’t think that she has nearly as much impact as the fucking leader of the KKK on social media.

Saying the term "racist" is "constantly in flux" seems to imply its definition is completely arbitrary, or that no commonly accepted or generally agreed upon definitions exist. Regardless of what else people may say it does or doesn't mean, I think most people would agree that the white supremacist views of David Duke could be considered racist.

>If I say I'm anti-racist, but disagree with you as to what constitutes "racism", which of us is right and which should be banned? Who gets to judge?

In the specific case, obviously the owner of the platform. Despite what people say (or want to be true) Twitter is not a public space, it's a private platform and its owners can ban whomever they please for whatever reason they want, and that's probably spelled out in their terms of service somewhere.

In the more general case, possibly a framework of law, common societal or religious norms and cultural influences, or no one. Truth can't always be determined with absolute certainty when one is dealing with social constructs in the subjective and abstract landscape of modern hyperreality. People can't even agree on the world being round anymore, and it's easier to accept that everything one is told is a lie than attempt to reconcile a complex and fragmented media universe of mutually opposed, violently overlapping magisteria. Obviously, if we can't even agree on what race is, we may not agree on what what racism is, or opposition to it. How do you even do that when some people believe most of the people they pass on the street are holograms in the Matrix?

That said, when David Duke talks about "the Jews" and their campaign of "white genocide" or about his desire to live in an all "white" society, it's entirely possible to see what he means in that context and form an opinion accordingly.