There is also an extremely detailed account of how to do this under Mailman on the FAQ. Which had the OP read, he would have needed to post frequently answered questions to this mailing list.
Should it really take a detailed FAQ entry to explain how to set a list open or closed? This is such a basic requirement it should be obvious. It is obvious in other list management software.
I run an announcements only list and find Mailman to be a breath of fresh air to use. If you don't, find another MLM to use, or code your own, and stop flaming Mailman.
I'm not flaming the list. I expressed some feedback which could be used to make Mailman even better. I want to see the best possible Mailman, and will contribute code if I can. But if the list admin interface is not intuitive and easy to use, then people are not going to use Mailman. That's not good.
Paul