Prevent users from logging into x site(s).

Hi Guys :)

I've been digging a little and haven't been able to find if this functionality is in Bakery or not, so this "issue" is more a question than a support request. I basically want to know, is there is a way to let users only create an account for/sign into the site that they signed up on, plus the default install?

I'll have a 6 multisite setup initially, but each of the multisites have a different function and as such users would not have anything to do on the other sites, which is why I would like to prevent them from logging into those sites at all. The default site install is more or less a hub for the rest of the sites, which I would like all users to be able to log into. At the moment, I cannot think of a use case where a user (except admins) would ever want to log into another of the multisites.

If anyone has any ideas on the subject, I'm all ears to those too :)

Cheers!
Josh