Plone: Control User registration

• login as a manager
• go to the zope management interface (ZMI) at the root of your site (i.e. http://www.my.portal/manage )
• look at the security tab
• against the Add portal member role ... uncheck the Acquire? check box .. so that permissions aren't inherited from Zope
• make sure only the Manager checkbox is checked
• save the form

Disabling new user registration from unauthenticated users this way will also automatically remove the "New user?" and "join" links from the portal interface.