silverstripe-cms/tests/model/SiteTreeTest_AdminDeniedExtension.php

<?php

use SilverStripe\Dev\TestOnly;
use SilverStripe\ORM\DataExtension;

/**
 * An extension that can even deny actions to admins
 */
class SiteTreeTest_AdminDeniedExtension extends DataExtension implements TestOnly
{
    public function canCreate($member)
    {
        return false;
    }

    public function canEdit($member)
    {
        return false;
    }

    public function canDelete($member)
    {
        return false;
    }

    public function canAddChildren()
    {
        return false;
    }

    public function canView()
    {
        return false;
    }
}