Using the signup feature on multisite, how can I disallow the listing of a particular site in a new user's site list (at the top left hand corner). As the multisite admin, I am using a site as my main entry point. When a new user registers, the user is automatically a member of the main site -- which is what I'm trying to avoid. The new user sees the main site in his dropdown list. When he clicks on it, he gets a 'You are not allowed to access this part of the site' which is ok but I'd prefer that this site does not show up in the new users' list.
I've tried removing this testuser from the site in the Network dashboard. This takes the main site off the list. However, if this is a user that has only signed up for an account (and no site), this doesn't appear to work. The main site still shows as the users' sole site in this situation.