I'd like to suggest providing homeserver admins the ability to shadowban one of their local users; that is, set a flag on an account that will cause the homeserver to auto-redact that user's messages from that point in time forward, with the exception that the shadowbanned user still sees their messages as unredacted and therefore cannot tell they've been shadowbanned. This would be extremely useful in dealing with persistent spammers and trolls because unlike current moderation tools (kick, ban, mute), it isn't overt and doesn't signal the problem user that they need to create a new account.

Because this would be a very powerful feature with potential for moderator abuse, and it additionally would have implications for server resource usage (shadowbanned users' messages are still stored and processed, which means problem users can still affect the homeserver), I propose that only homeserver administrators would have the ability to shadowban accounts on a per-server, rather than per-room, basis.

I'd really like the feedback of the Matrix team and our administration and moderation community on this feature request. If there's general agreement that shadowbanning would be useful to have and the semantics of the feature are decided by the community, I will be happy to implement the feature and submit pull requests.