I researched this for my "Configuring Microsoft Outlook 2003" book. There are several steps you need to take to suppress stationery completely:
1) Create a NewStationery value in the HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0\Common\MailSettings key and leave it blank to prevent users from choosing any default stationery. You can create a new .adm file if you want to add that to the group policy editor.
2) Use the group policy editor to disable certain toolbar commands in both Outlook and Word:
Outlook
Actions | New Message Using More Stationery 5611
Word
Format Background 30403
Format Theme 3623
Format | Background Standard Colors 3224
Format | Background More Colors 2857
Format | Background Fill Effects 2858
Format | Background Printed Watermark 4003
View HTML Source 3902
For a primer on setting group policies with Outlook, see
http://www.howto-outlook.com/howto/policies.htm
--
Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers