You can block communities that attract problem users.
You can even block entire instances of communities that attract problem users (on your own, without the instance choosing for you).
The Fediverse isn’t completely unmoderated, but it’s not hella huge either. Plenty of moderation issues due to lack of having mods available 24/7 already exist. Just a few weeks ago, CSAM was being spammed all over the place and it took almost an entire day for action to be taken.
I for one am not in favor of giving my ability to curate my experience myself up to have someone else choose for me, so in almost every case I would be against defederation from the admin level.