How do I stop ALL my subscribers posting to my list?
In your list's Admin section, under 'General options', set 'Emergency moderation' to 'On'. (This is about half way down the page you land on when you login to your list's Admin section).
Once Emergency moderation is on, all messages posted to the list will then be held under 'Tend to pending moderator requests'. You can then choose to accept or discard the message.