I've been able to use disabledcmdbaritemslist to prevent users from changing the fonts in Outlook, and have also entered the registry keys to use a specific font. In addition I've created a dummy Theme and is forced through GPO to prevent users from changing their stationery or theme. 

However, now when a user needs to add or change their signature, the font is defaulting to Calibri-11. What registry is used to force the font in the signature window within Outlook?

Seems there might be no way to prevent users from changing their stationery or theme in signature setting. However, we could consider to define their signature and then disable the Control ID of creating signature for workaround. 

Tony Chen
TechNet Community Support

• Marked as answer by Tony Chen CHNMicrosoft contingent staff, Moderator Tuesday, April 15, 2014 5:59 AM

Can you please tell me how you were able to disable the fonts in Outlook using disabledcmdbaritemslist? I would like to set it so that the Fonts button is grayed out in the outlook options window. Or if they change it it automatically reverts back to what I want it to be as soon as they click OK.