A remote user that is banned from this instance will also be banned from the communities on this instance, and those community bans federate, so they can't interact with remote versions of the community.

They can still see the content in the community though. I think there is a gap around communities that they had never interacted with. They can't subscribe to those communities because of the instance ban, but I think in theory they can still post to them, but that content will only be visible on their remote instance. I'm not certain of the details of this particular edge case though...