To mark a courtesy car as unavailable you do the following:-
Go into the Service Managers Toolkit and select the Courtesy Car option.
Double click the required vehicle to mark as unavailable. The below window will then appear:
Enter the to and from date in the field for Unavailability and add a note explaining why the vehicle is unavailable.
This will then mark the vehicle as not available on the courtesy car diary. The vehicle will not be allowed to be booked to a workshop booking for the dates that were selected.