Hi,
We can use Group Policy to disable most Outlook menu via using the Control ID. For the "Mark as unread" right-click menu, you can use
1675 to disable it.

Here is the effect after applying this GPO setting:

See the following article for a detailed guide about how to use the Control ID:
https://technet.microsoft.com/en-us/library/cc179143.aspx
Hope this helps.
Regards,
Steve Fan
Please remember to mark the replies as an answers if they help and
unmark them if they provide no help.
If you have feedback for TechNet Subscriber Support, contact
tnmff@microsoft.com.