If you wish to prevent users from using Microsoft Word to create Outlook email messages:
NOTE: This procedure varies slightly based upon the version of Outlook and Word that you have installed.
1. Use Regedt32 to navigate to:
NOTE: The 9.0 in the above registry key path is the version of Word.
2. Double-click the default value name,
3. Select the OutlookEditor key.
4. Use the Security / Permissions menu to set the users permissions to Read, only.
5. Exit Regedt32.
When the user tries to change this option, it will be grayed out.
This tip was provided by Kevin Costain.