It is incredibly easy to defeat white supremacy in a debate. Just point out the arbitrary deficiencies of certain members who are generally agreed to be white.
Done.
If that were the case there wouldn't be that many white supremacist to be a problem.
It's easy to "defeat" them in the sense "Expose their logical fallacy so that most reasonable people can see it". Unfortunately, it doesn't do shit. So, in a sense, debating a white supremacist (and winning logically) is a pyrrhic victory, something that gives satisfaction to the practitioners but unlikely to make any difference in the society, at least not the kind you'd usually expect from "winning a debate".