This has been tried before in different ways; infowars, parler, even giving them r/thedonald (as some believed it kept them off other reddits) ... It has only led to fascists finding a platform to start working their poison and spread it outside.

In fact isolating only worked when moderators finally did something and deplatformed them, I.e. Banned these communities. After Alex Jones was taken off youtube, infowars died. Spencer was punched so much he stopped leaving the house (and so stopped propagandising). Milo was canceled everywhere he went (literally), but especially on Twitter, and last I heard he filed for bankruptcy.

The best way to isolate them is to fragment their communities so much that any organisation is impossible. And of course prevent them from creating such communities. If you let them have their spaces, they will find them, they will go on there (and they're very good at using the edgy aspect to lure in new recruits), and then you end up with another terrorist attack. They will organise on there and that's what you want to prevent.