How to prevent users from editing specific pages?

On our MU install, we have several users that have pages with some scripting or certain bits f code in the WP Editor, and we don’t want them to be able to edit those pages, because the visual editor breaks them, and causes us nightmares.

How do we prevent users from editing pages on their own sites, either whole cloth preventing page edits, or specific pages?

I’m sure this will require a plugin for permission management, but we wanted to ask here because we need it to work with Mulituser.