How to override logic of specific drush command?
Actually returning just FALSE without drush_set_error should work fine without error and the original command isn't executed, e.g.

/**
 * Validate feature revert of specific module.
 *
 * Implementation of drush_hook_COMMAND_validate().
 *
 */
function drush_policy_features_revert_validate() {
  $args = drush_get_arguments();
  if (strpos($args[1], 'rules') !== FALSE) {
    drush_log(dt('Reverting rules...'), 'ok');
    drush_invoke_process('@self', 'rules-revert-all');
    drush_log(dt('Reverting of rules finished.'), 'ok');
    // drush_postflight(); // Set context as completed, but this is not needed.
    return FALSE;
  }
}