Disable sending read confirmation in Outlook 2011

When one first receives a mail with read confirmation in Outlook 2011 the question appears to either send or don't send the read confirmation, with an additional option to not show this again in the future. One of our users clicked to send the read confirmation and selected the option to not show this again, so now everytime he receives a mail with read confirmation it is automatically sent. This seems to be irreversable because I can't find where to disable this.