I'm not going to offer a specific solution since others have and I haven't read that much theory regarding anarchist justice, but I would like to point out that overdoing things by any means would backfire badly. I can't think of anything people could do to inflame a counter-revolution than going on witch hunts and arresting people with low standards of evidence. Continue to pursue things overzealously, and you could easily fall into a trap of paranoia and end up doing stupid shit like with Stalin and the Doctors' Plot.

You win by giving people freedoms that they don't want to give up. Anything that's perceived as taking freedoms away will end up being a liability. Because of this preventing a counter-revolution would largely end up being a propaganda war. Gulaging any amount of the wrong people is an easy way to make propaganda material for the other side, but if it happens to people who are very clearly threats it would show the necessity of whatever system -- with the caveat that this will make it harder to abolish the system down the line.

Actually, now that I think of it, having people infiltrate counter-revolutionary groups would likely be somewhat easier under a decentralized system. Having no agency, you could just have people with no real background to discover under a call to arms to infiltrate, disrupt, and expose white supremacist groups, and having it being openly organized and widespread would add a lot of overhead to counter-revolutionary organizing.