How can I force a specific password?

You can use this code in your functions.php to restrict users below admin level from changing their passwords:

if ( is_admin() ) {
  add_action( 'init', 'disable_password_fields', 10 );
}

function disable_password_fields() {
  if ( ! current_user_can( 'activate_plugins' ) ) {
    $show_password_fields = add_filter( 'show_password_fields', '__return_false' );
  }
}

The admin should probably register each user manually if possible and select a strong password for them.

Edit – Changed user level check. Syntax.