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Piefed has one built-in, Voyager has it client side, there are probably other apps too, feel free to share your list in the comments
(i.imgflip.com)
Seems like a good reminder as this topic shows up regularly
Lemmy's user-level "instance block" does not block the instance though, i.e. it blocks neither users nor content from the instance, e.g. Lemmy still pings you if a user from that instance replies to your content in communities located on another instance. It would have been better named as a "community mute" instead, as all it blocks are the communities located on those instances (regardless of the origination of the post itself).