There are hacky ways to do it through a frontend that can only block communities. Any time a post appears from that instance, automatically block the community and hide the post. I don't know if jerboa would prefer to wait until a more elegant solution is possible and/or leave it to the user though.
If I built it, I would just filter it out as it comes in. So don't send anything to the server, just hide stuff. That seems like the least surprising behavior.
So that you don't need to seek out "which instance isn't federated with [instance I would wish to defederate from had I the ability]" and can instead simply click "block instance." And could do this without hosting my own whole ass instance.
Say another NSFW instance pops up and gets really popular...but it's only furry and futa, and a lot of it, and new communities all the time pop up for vore, and guro, etc. Sure, one can whack-a-community everytime one pops up if they don't want that being prominent in their all tab, or they can just click "block instance" and keep it pushing. It isn't something bad enough I feel I could (nor should) bother my instance admin about defederating, and if he disagreed with me I'd like to not have to create a whole new ass acct just to not see c/specificguro pop up on my shit after banning c/guro and c/leftguro, and I don't want to block all NSFW because A) It isn't always porn, that tag is sometimes used erroneously, and B) I'm cool with some titty, but cockvore is a little much.
I dislike censorship more than I dislike having to constantly see posts by tankies and nazis, but only barely. I don't want to have to advocate for defederation from obvious loony instances if there's a possibility to just block them individually. There's just not a single user or community I am interested in reading the opinions of with @lemmygrad.ml in their handle - it's a perfect heuristic. So why not allow me to use it?
Also I'd love to be able to filter by entire instance (besides "global", "local", and "subscribed", if we could add instances where we can only see content from that instance)