by Lawrence Ng.
Calvin,
You can control this
Under Site administration -> Plugins -> Authentication -> Manage authentication
Check off the "Prevent account creation when authenticating"
As described ....
"When a user authenticates, an account on the site is automatically created if it doesn't yet exist. If an external database, such as LDAP, is used for authentication, but you wish to restrict access to the site to users with an existing account only, then this option should be enabled. New accounts will need to be created manually or via the upload users feature. Note that this setting doesn't apply to MNet authentication..."
Hope this helps