How do I remove top bar access to master site for simple users in WPMU ?

Hi there,

Sorry, I'm asking a lot of questions these days, because I'm working on a big important project and the deadline is next Monday.

I have a WPMU+Buddypress community where users can create their own child site. There is just one problem :

When a simple user access his account, in the admin top bar he of course has access to the "My Sites" menu, within which there is his site's name.

However, he also has access to the master site. He doesn't have any specific access but he can see it and access an "empty" dashboard (since he has no rights to the master site).

More specificaly in my case, look at the below picture :

https://dl.dropboxusercontent.com/u/5140132/wpmu_master-site-why.png

As you can see, a simple user created his account and website called "Dushan Test 4", which is fine and dandy. BUT he aslo can see the master site (called "AGenève" in that case). That access is of no use to him since he has no rights to edit or do anything on that master site.

I think it's confusing, so what would be the best way to make it so the simple user doesn't see the master site for no reasons ?

Cheers,
DJ