It takes just two simple steps.
Firstly, on the 'Add tickets' section of the event editing page, create a ticket type called ‘Comp’ with 0.00 as the price
Then, click ‘Options’ on the right and in the drop down area, change the option for 'Which outlets can attendees buy tickets through?' to 'Just through the box office'
That way, customers won’t be able to buy comps but you will be able to allocate them through the box office