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I don't think you can block whole instances, but you can block communities. Open the community on your local instance and click Block community at the top of the sidebar, just below the subscribe link.
https://lemmy.one/c/[email protected] <- here is the local link, but you might need to block a few communities if they are spread out.
I think this is correct, but I'm not 100% certain...
To defederate from an entire instance, in this case lemmynsfw, click the triangle-looking icon (beneath top bar, towards the right side of the page) and then set Status to Off.
To block just one of the categories of lemmynsfw, on the right side, scroll down just a little to see Magazine, then click the "no" icon for block magazine.
This probably works for kbin but OP is on lemmy.one
The triangle-looking icon is for turning ALL federation off, not blocking an instance.