Disabling Outlook Anywhere for External Users with Exchange 2013

For security reasons we need to disable Outlook Anywhere for our external Exchange 2013 users.

Since Exchange 2013 explicitly uses Outlook Anywhere for internal and external Outlook connections using "Set-CASMailbox -id <user> -MAPIBlockOutlookRpcHttp:$True" is not an option as it will break all Outlook connections.  Also, nulling out the external Outlook Anywhere hostname does not seem to work since we don't use ambiguous namespaces for internal and external connections.

We are looking into the option of totally disabling our external autodiscover setting, but I believe if the user still knows the server information they will be able to make a connection anyway, plus we would like to use autodiscover for ActiveSync.

Any help on this would be greatly appreciated.