Erm... create a role with no capabilities? On a more serious note, the roles
system doesn't have an 'access_admin' capability.
You will need to set up a redirection:
function no_admin_access() {
if ( is_admin() && current_user_can( 'a_role' ) )
wp_redirect( get_bloginfo('url') );
}
add_action('init', 'no_admin_access');
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