To make an announcement only list please do the following:
1. Go to Membership Management...> Membership List
Set "Set everyone's moderation bit, including those members not currently visible" to on. Its under the "Additional Member Tasks" section of the page at the bottom.
2. Go to Privacy Options --> Sender Filters
Do the following:
- "By default, should new list member postings be moderated?" Set to yes
- "Action to take when a moderated member posts to the list." Set to discard
- "List of non-member addresses whose postings should be automatically accepted." Put the e-mail address that you will be sending from to the list address here. Another alternative is to add the sender as a non-moderated member to the list.
- "Action to take for postings from non-members for which no explicit action is defined." Set to discard
Your list is now configured to be a newsletter only list.