Administrators are able to set an Agent's Permissions to prevent them from making changes to a Booking once it has been set to Invoiced.
Under Settings>Users, select the Agent you wish to edit.
Go to Permissions>Bookings
Change the Edit Invoiced Bookings permission
Once the permissions has been edited, this notice will display above all Invoiced and Closed Bookings:
as well as disabling the following controls in the booking: