How to delete all posts for an admin

Users listed in the .yml build file as DEVELOPERS cannot be deleted from the UI for security reasons:

 ## TODO: List of comma delimited emails that will be made admin and developer
  ## on initial signup example 'user1@example.com,user2@example.com'
  DISCOURSE_DEVELOPER_EMAILS: `incredible_undeletables@myamazingdomain.com'

You will need to remove them from the .yml build file and rebuild.

Alternatively, but not recommended and not supported (I assume), is to change their admin status (boolean) in the users table for that user to FALSE, directly in the database. Note: I have never done this (never revoked, but I have added moderators this way), but doing things like this (tinkering directly in the DB with things you are not totally familar with) could have unintended consequences, so this is not advised unless all else fails (and make sure 100% you have a working backup of the DB before you do!).

Check your .yml file… the issue is more-than-likely there.