Options for restricting access to wp-admin

You could make it a mu-plugin (‘Must Use’ plugin). Any PHP file you put into /wp-content/mu-plugins/ will automatically get included in WordPress. You can’t deactivate the plugin (unless you have ftp access to the server). If you go with a mu-plugin, make sure to put the functionality into a subdirectory and bootstrap it with a php file in the mu-plugins directory.

EDIT

After reading the comment, I think I understand the problem better. It sounds like you want to be able to lock people out of the admin altogether. That’s not so difficult. Try this:

function my_awesome_admin_lockout(){
  if( is_admin() && !current_user_can( 'manage_options' ) ) {
    wp_redirect( home_url() );
    die();
  }
}

add_action( 'init', 'my_awesome_admin_lockout' );

Basically, that locks everybody but admins out of the admin area.