Is there any way we can block internal and external autoreplies in Exchnage via Exchange / GPO? Our users sometimes setup out of office using rules (instead of the default out of office) and these out of office messages some times create a mail loop for our IT Helpdesk system. We are looking at some way ti block users from setting up auto replies.

An auto-reply is just an Outlook rule. I would ask in the Outlook Forum to see if you can disable the ability to create an Outlook rule ( And that would mean any rule) http://social.technet.microsoft.com/Forums/en/outlook/threads