They do not need to listen (they cannot be forced to listen), although a federated service should not prevent federation with an instance (even if they hide som

They do not need to listen (they cannot be forced to listen), although a federated service should not prevent federation with an instance (even if they hide some or all messages) just because one message that has been added there is what the users of the other instance do not want on their computer. Blocking it entirely will just prevent communication even among the users (not the administrators) who wish to communicate freely, and will just result in more ignorance instead of better education. People could still vote against a message and apply tags and whatever, and can prevent messages from specific users from being displayed on their computer if they wish (by using scoring files (like on Usenet) or otherwise) (although they might be missing something), and even delete one copy of it (although it is best that users delete their own local copies instead of everyone else's), but you should not prevent a message from being accessed (or from being sent) at all costs.