I don't want new users adding as subscribers.

Hope someone can help me, or point me in the right direction.

I manage a Multisite blog authenticated by LDAP, users don’t create accounts as such, we direct them to the http://domain/wp-admin, they enter their LDAP account details, the LDAP plugin creates an account for them and they become a subscriber of the main site.

From there they can create their own blog etc…

The problem I’m having is that some of our blogs use the WPMU ‘Multisite Privacy Plugin’ to restrict access. Blog administrators who choose the option ‘I would like my blog to be visible only to users I add to “ABC Blog” are getting these settings bypassed by users who don’t have an account on the system, but are creating their accounts by going to http://domain/abcblog/wp-admin. Because their account gets created there they get added as a subscriber to that blog and they get access to it.

I need some kind of plugin or code that removes access to the first blog that a user gets access to on account creation so they sit in a state of limbo. A. they either create their own blog or B. a blog administrator manually adds them to one.

How easy would this be?