I need to disable the ability of the end-user to modify Stationery and force a default stationery and font. I have found the "font" forcing but not the rest. I was trying to work with control ID but don't know how to find the right ones.
You'll need to make at least three changes:
1) The registry value that controls the default stationery for the user is in the HKEY_CURRENT_USER\Software\Microsoft\Office\nn.0\Common\MailSettings key (nn is your version of Outlook) in an expandable string value named NewStationery. Set the value to the name of the HTML file, without the .htm tag. In other words, if the name of the stationery file is "Company Letterhead.htm," the value for NewStationery would be "Company Letterhead." If you put in the name of a stationery file that is not in the user's Stationery folder, e.g. "dummy," Outlook will use the user's default fonts, and the user will not be able to invoke any default stationery.
2) In the same key, you should also create a string value named NewTheme to a dummy value to block the user from using Word's Tools | Options | E-mail Options dialog to set a theme.
3) You would also want to disable the Actions | New Mail Message Using | More Stationery command using a group policy object and the "Disable command bar buttons and menu items" policy setting or edit the registry at HKEY_CURRENT_USER\Software\Policies\Microsoft\office\xx.0\outlook\disabledcmdbaritemslist. The ID for More Stationery command is 5611. The Options | Mail | Stationery and Fonts button ID is 3766.
Outlook needs to be restarted for new group policies to take effect.
However, these measures will be only partially effective in enforcing the use of particular stationery. The user could always edit the raw .htm file in the Signatures folder or replace it with one with the same name. A WordMail user can also change the background and fonts by using the Format | Theme command, unless you block that menu choice for all Word documents. Furthermore, if the user has previously used stationery, the most recently used stationery choices will be available in the Actions | New Message Using menu and cannot be cleared. I haven't found the MRU list yet.