If it's known in advance the flights aren't going to operate then the slots can be cancelled with ACL. Once you cancel the slot or slot pair with ACL, they inform the airports and the flights are removed from the flight screen database. If a flight is active (usually a day or so before ) in the AOS system then the only option left is to show it as cancelled, otherwise AOS will plan stands etc for it. A good example of this is on bank holidays or at Christmas where you will see some airlines with a lot of cancellations on these particular days while others despite operating a reduced schedule don't have any showing as cancelled.