How shall we deal with such hard-core Neo-Nazis and White Supremacists?

Martin Luther said it:

“The best way to drive out the devil, if he will not yield to texts of Scripture, is to jeer and flout him, for he cannot bear scorn.”

If they aren't responding to reason, mock them. Laugh at them. Point fingers and giggle at them. Tell jokes about them. Reduce them to the butt of humor, both juvenile and witty.

https://www.nytimes.com/2017/08/17/opinion/how-to-make-fun-of-nazis.html