Moderating open organizations

An open organization is one where anyone can join without an invitation. Moderation is a big part of making an open community work.

Prevention

Disallow disposable email addresses or require users to log in via GitHub.
Restrict who can create streams, create bots or add custom emoji.
Link to a code of conduct in your organization description (displayed on the registration page).
Create at least one default stream where only admins can post.
Add a waiting period before new users can take disruptive actions.
Restrict email visibility to reduce the likelihood of off-platform spam.

Ban (deactivate) users acting in bad faith. You can reactivate them later if they repent.
Use the streams:public sender:user@example.com search operators to find all messages sent by a user.
Delete messages, delete streams, and unsubscribe users from streams.
Rename topics.
Change users' names (e.g. to "Spammer")
Deactivate bots or delete custom emoji.
Instruct users to collapse messages that they don't want to see.

Delete spammer. This will wipe the user from your Zulip, by deleting all their messages and reactions, banning them, etc.
Mute user. This will allow an individual user to hide the messages of another individual user.
New users join as guests. This will allow users joining via open registration to have extremely limited permissions by default, but still enough permissions to ask the core team a question or to get a feel for your community.
Public archive. This will give a read-only view of selected streams, removing the need in some organizations for having open registration.