So block them and move on. Or if you disagree with them that badly, push to have that instance/community added to a public blocklist/filter. Defederation, besides being an overly authoritarian solution, damages the network in a way that can and will make Lemmy into a worse place for its users.

As instances start to defederate, it will matter more and more which instance a user signs up to. This will push users towards larger instances. As instances get larger, they will become less and less reliant on 3rd party instances for content, those instance admins will be incentivized to defederate from them, as they will a) not have as much control over those instances, and b) start to view them as competitors rather than collaborators.

The beauty of Lemmy and federation generally, is that information appears centralized to it's users, despite being decentralized in reality.

The more defederation is used, the more centralized Lemmy becomes, whilest giving a more fractured, confusing, and disorganized experience to its users.

Defederation will kill Lemmy. It needs to removed from the protocol before it becomes too widely abused.

I see Lemmy going down in one of two ways:

In fighting and fragmentation with overzealous use of defederation leading to walled gardens, and a terrible user experience.

Or

A major player like google or microsoft sees the potential in Lemmy. Starts their own highly funded instance that is full of useful features and a wonderful smooth UX. (That is all proprietary and only usable on their instance, naturally) Then when the majority of users are on their instance, defederate from everywhere else. (If you don't think this can happen, just look at what google did to xmpp).