When a new user account is created from the admin's dashboard, I see in the code that the form's action is "wpmu-edit.php?action=adduser". So I found wp-admin/wpmu-edit.php and the case that matches adduser. It simply has this line in there:
add_user_to_blog('1', $user_id, 'subscriber');
which I then switched to:
// add_user_to_blog('1', $user_id, 'subscriber');
Then saved, and went back and created a new user account from the admin's dashboard. The new user still got access to a blog, the default one. I don't want new users to get access to any blogs when they first get a new account, I want them to have to go fill out the form to get one. Any ideas on this one and how to shut it down?