deny access to the BACKEND for people who log in (in 1 of 10 mu-blogs)

hallo,

we are running a multiuser-installation including approx. 10 blogs.

for exactly one of these 10 blogs, we need to set up a very special (?) password protection.
this needs to be a user/password combination which will be emailed to several hundred people. it is not an option, to set up an account for every single person.

what’s special: we must avoid access to the BACKEND for the people who log in.

can anybody give me a hint how this can be done?

we know the plugins…
More Privacy Options
http://wordpress.org/extend/plugins/more-privacy-options/
…and…
Authenticator
http://bueltge.de/authenticator-wordpress-login-frontend-plugin/721/

…which do the first part of our task – access only with password.

now I wonder how to deny access to the backend.
could something like this in function.php help?

get_current_user_id();
if (user-id = ...) {
 redirect
    any path containing "/wp-admin/"
    to frontend
}