Code has since been added to WPMU core to disable registration/blog creation at SiteAdmin-->Options. But the options do not limit blogs to logged in users while keeping registration of new blogs open.
Try simply removing the "signup_another_blog" from wp-signup.php
if ( is_user_logged_in() && ( $active_signup == 'all' || $active_signup == 'blog' ) ) {
// signup_another_blog($newblogname);
_e( "Only new users may create new blogs at this time." );
}
Consider: once any user is assigned to any role(subscriber-editor) on any other blog, they will in effect have multiple blogs and can choose any other blog as their primary blog. Any blog becomes theirs if the existing admin promotes a user to admin, and they switch primary blogs in their profile.