WordPress MU

Apparently when an orphaned user (username and password but no blog -- so sad) tries to log in, WPMU kicks them into "subscriber" status on the main blog. I don't want this to happen.

I need the orphaned user to just not get logged in and instead get sent to the "user does not exist" or even a custom "you are not welcome here" page.

The reason being -- when integrated with bbpress, I want user to be able to have bbpress accounts, but NOT wpmu accounts. So they will exist in the DB... fine, let em, but "Stay out of Malibu Lebowski!" stay out of WPMU you deadbeat!

Or is this waaaay too deeply ingrained into the functions of WPMU?

Oh - and going the other way is fine... All WPMU users will get bbpress accounts.

And:

WPMU: 2.8.1
bbperss: 1.0.1

Can we maybe define a new user role... a lower-than-low... like a "ghost" role?