WordPress MU

I am decidedly content with users being Admin's of their blog, and having "-- no role for this blog --" for the main blog.

However, bbpress keeps changing those roles (through shared user db. Yes, I know that is a bbpress issue, and I/we are working on it but bear with me.)

On the bulk actions for users, we can assign them any role for the main blog, except for -- no role for this blog --. Can anyone point me to where/how I should add this option?

Or barring that -- is there a way to prevent users from EVER visiting the main blog backend unless their role Site Admin. Even if their role is set to "Contributor" "Editor" "Copy-Boy" etc.?

No listing on the "my blogs" button, no wandering in by accident, never, never, never...?

Thanks all.