How to close a Plone Site

If you do not plan to have untrusted users register as members for your Plone site, it is a good idea to close it. When this is implemented, only users with the Manager role will be able to add new users to your Plone site. Otherwise, by default anyone can register as a new member for your Plone site.

http://plone.org/documentation/how-to/closing-site