You can already basically do that by just not using any of the relays Mostr pushes things to. There's a list of them. But the worst cases I saw weren't on Fedi, they were on Nostr.

Some of the devs were talking with Alex the other day about an NIP that could theoretically do instance mutes, rather than Mostr as a whole. That will take care of things on that front if it ever happens.