I mean, I am not fond of the head admins running Lemmy.ml, but you should just do what I do and block the instance. Don’t know what you use to browse Lemmy but I use Connect for Lemmy on Android and it supports instance blocking.
No reason to prevent other people on your instance from interacting with instances they want to. Not everyones ideologies have to match your own, and different people want to see different content.
Instance blocking in Connect for Lemmy also hides all comments from all people in the blocked instance. It doesn’t make them disappear like blocking a user does, but the comment appears with the body text replaced by a button that allows you to optionally unhide the message (like how Discord used to hide messages from blocked users).
Defederation should literally be a last resort option IMO. Otherwise you end up like Beehaw: isolated on an island away from like 90% of the Fediverse. Great for moderation but terrible for discoverability and growth.
Lemmy is pretty much the Linux version of Reddit, and as someone who has used Linux before, it is up to the end user to do all the legwork with Linux as opposed to something like Windows which is pretty much plug and play. Lemmy does best when it gives as many options to its users as possible, including preferring local user instance blocking as compared to defederating.
As an example, I like anime. Let’s say hypothetically that you don’t like anime and that I have an account on your instance. And let’s say you hate anime, actually. You want your instance to defederate from ani.social (and any other instance that hosts anime content) because you hate anime, but I don’t because I like the memes and discussions from there. How do you resolve this? Now think about what happens with an instance that hosts content that is of a different political ideology than you. Its really the same problem. By calling for defederation you effectively say that your opinions and viewpoints are the only correct ones, and that everyone else on your instance of choice needs to align to your viewpoint. In this case, this is why it would be better for you, the user with the problem with those instances, to locally block that content.
I can understand if a particular instance is so full of spam bots or people that attack the instance with illegal post content, such as posting CSAM anywhere and everywhere, but in those cases it would be up to the instance admins, and a responsible admin will make a post explaining whats going on and in some cases even ask for the community to give input on it.