XenForo_Helper_String::bbCodeStrip() is called only by the {xen:helper, snippet, $text} template syntax function.
The functionality of this function can be overriden with a callback using the init_dependencies code event, with a static function like this:
PHP:
public static function addTemplateHelper()
{
XenForo_Template_Helper_Core::$helperCallbacks['snippet'] = array('MyAddOn_MyClassName', 'bbCodeStrip');
}