Federation can get quite qickly quite philosophical, especially on the question when/what/how/why to block.
On our Hubzilla-Instance ‘Chatsubo’ there are many defederated hosts and a few blocked users.
We defederate whole hosts when most of their posts we get to see contain obvious rascism, antisemitism, alternative facts…
We simply defederate through the admin-interface of Hubzilla.
Why? Is the most interesting and philosophical question on the issue.
Believe it or not, there are users who create new, federated account(s) every day(!), just to nag and stalk and piss of others. looking at you, zlax! It’s a bit of federated common work to track these accounts, and on some servers (on Chatsubo, too) he is simply deleted on database-level.