If they march peacefully, disgusting as they are, they have the right to march. If they become violent, then let the authorities deal with them.
We have some real nutjobs out there on both the Alt-left and Alt-right. Sad but true. People should not confront them. Let them make their noise, pay them no attention, then they will likely leave peacefully.
The public and the media should ignore them. Why legitimize them? I think the media frenzy only adds to the stupidity.
Do you remember the Westboro Church, they protested veteran's funerals? A group very outspoken against the LGBT community. They've pretty much dried up and blown away. The same will happen to these other groups in time.
FWIW, the Klan's numbers are much lower than years ago. Their significance is thankfully in decline...