Well here is the thing once you allow the powers that be to shut them down you are doing 2 things.
One you are legitimizing them. They are saying that they are not being represented, and are crying fowl you will only give them more publicity. If Antifa,, and all the other counter protesters didn't show up to start trouble nothing would have happened in the slightest there were something like 500 people that showed up (OOO big bad scary protest). No one would have heard about this outside some local station reporting it. In all truth the media, and the other groups sorta played right into what they would wanted. Lets face it if a crazy man is spouting idiotic things on a box do you put them on national TV so their idiocy can be heard by all?
Second is you are setting a dangerous precedent by restricting freedom of speech, and giving the powers that be a new tool to silence those they don't agree with. Today it might be who you deem Nazi's, and white supremacist. Tomorrow it could be anything you say that goes against what they deem dangerous. We have seen this with Antifa they started "punching Nazi's in the face", and that violence seems to have expanded to anyone to the right of Lenin.
As Evelyn Beatrice Hall once wrote "I do not agree with what you have to say, but I'll defend to the death your right to say it."