Block requests to change the publishingMode setting from anyone except the site admin.

block-publishing-mode.php

<?php
/**
 * Refuse requests to change the publishingMode setting from anyone
 * except the site admin.
 */
HookRegistry::register('Context::validate', function($hookName, $args) {
  $errors &= $args[0];
  $props &=$args[2];

  if (/* user is not an admin */) {
    $errors['publishingMode'] = ['You can not modify this setting.'];
  }
});