Yes, if you load up the Outlk11.adm administrative template file from the Office Resource Kit in the group policy editor and browse to the Security settings, you'll see that you can block all kinds of account types and prevent users from messing with their profiles at all.
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers