Restrict Access to Dashboard for P2 Themed Blog

I have installed WPMU with subdomains for sub-blogs. Using the site wide privacy plugin, everything except the homepage are restricted to logged in or registered users only. I also have buddypress installed. I created a sub-blog with a P2 theme to represent like a guestbook, meet and greet, twitter-like, etc. page.

Using an mu-plugin from scripts I gathered somewhere, all new registrations are automatically registered or added to that sub-blog as an author, so they can post right away from the front-end/homepage.

Now, I just wanted these authors not able to access the dashboard of the blog, just to avoid confusion, especially when they are already registered as admin to one or more blogs.

Let me give you some examples to make it clearer what I want to do. I created a blog under http://twitboard.example.com. Now I want the authors of these blog not able to access any of the pages within http://twitboard.example.com/wp-admin/. But of course, they can access the dashboard of their own custom blogs (e.g., http://myblog.example.com/wp-admin).

Now, the question, is there a script I can utilize to make this happen. Or maybe I should do it in htaccess (should it be even possible)?

I don't mind manually disabling and enabling back the script from time to time, just in order for me, the site admin, to make some changes from the wp-admin of that sub-blog.