Probably Joomla does not use the standard PHP Mail API, therefore "disabled_functions" does not have any effect on Joomla. Ask Joomla support how to disable mail.
carsten888 wrote:I want to make sure xampp is not sending any emails at all.
This cannot be done with a simple setting in php.ini, as there are thousand of ways how to send emails. You might configure a firefall to block all traffic on outgoing ports which are used for mails. Like port 25, 465 and 587.