Until we let them speak by marching in the streets, give them media coverage, normalise their behavior, allow it to spread, and then have a white supremacist drive his car into a block of protestors. You don't give hatred a platform, you force it into the shadows where it belongs, because the moment you let it out into the sun all it does is spread.As Evelyn Beatrice Hall once said: "I disapprove of what you say, but I will defend to the death your right to say it"
As you said, /pol/ and r/theDonald are hives of nazi assholes, but they are just that, stupid people being stupid to show other how stupid they can be. They are just making a fool of themselves.
The best way to discredit an jackass is to let him speak, he'll do the hard work for you.
"Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them" - Karl Popper