There may be instances where administrators do not want agents adding to or modifying bookings once they have been invoiced.
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Administrators are able to set this as part of their user permissions under Settings > Users.

Select the user you wish to edit, and from the Permissions tab, de-select the Edit Invoiced Bookings permission in the Bookings dropdown:

Once this has been set, the following notice will display above all Invoiced and Closed Bookings:

The following controls in the booking are also disabled at this point: