I posted about this issue a week ago:

Currently, instance bans and community bans are treated as two separate things. When a user is banned from an instance, you’ll often see in the logs a bunch of community bans alongside it at once (at least from Lemmy communities). These are communities that user has posted on. An instance ban automatically applies hard-bans to communities they have interacted in from that instance. But the problem here is its only communities they’ve interacted in.

The instance ban itself is simply a rejection of federation. It doesn’t block users from posting in communities on that instance - only the community bans do that. It just means their posts won’t federate out. This means that an instance banned user can continue to be a nuisance in most communities (or all, if they are pre-emptively banned) on an instance locally - and the moderators of that community and instance won’t even know because they don’t view their community from there. With larger numbers of users would also mean larger amounts of trolls and incompatible users, which could greatly increase the chance of people simply vandalising communities and no-one even noticing.

Lemmy 1.0 promises to fix this apparently from their end, but I think at least for as Piefed is concerned we could get in on this first. We need a hard block on all Piefed accounts from being to interact on any community that is from an instance they are bannedfrom. We also need a way before that for Piefed based communities to automatically throw out all comments made even locally by instance banned accounts based from Lemmy.

It’s now been implemented on the latest piefed. A Piefed user instance banned from anywhere will now be hard-banned from all communities on that instance to prevent them from being a local menace unbeknownst the moderators of those communtiies. But all Piefed instances need to update.