Unfortunately fascist spaces will exist and cannot be prevented because of the core nature of Lemmy/fediverse architecture. Anyone can start their own instance and post whatever they want.

However, if the main popular instances defederate with them, then most users won't have to see those posts.

Umm, you're aware that anyone can stand up a Lemmy instance? Communists and fascists alike, you can just defederate from any instance you don't like.