Block Outlook Web App (OWA) 2013 for External Users

Hello, 

I would like to block the Outlook Web App (2013) externally, should be accessible internal only. I have two Client Access Servers, one facing externally and one facing internally for administration purpose. I have been looking at the options to block the external facing CAS for OWA only. I referred this article: http://blogs.msmvps.com/expta/2013/09/17/how-to-block-owa-2010-and-2013-for-external-users/, it doesn't help me much. Anyone had luck implementing this? 

Thanks!