I don't believe there's a "disable all registrations" switch, but if you go into the site map (
http://xyzzy.com/admin/site-map/) then set parameters for the root site, then to the user-login section, you can set the RegistrationRequiresApprovalP parameter to 1 to require that all new registrations be approved before they become active. You probably also want to make sure NotifyAdminOfNewRegistrationsP is set, and NewRegistrationEmailAddress is your email address.
http://xyzzy.com/acs-admin/users/ has a "Find all users needing approval" option where you can see all pending registrations and approve/reject them.
cheers
