This example will show you how you can cancel the otherwise valid discounts in a particular period.
Open the new discount like you normally would, but in this case leave the Discount name (1.) empty so that it does not show. Next you set the Trigger type to Service period (2.), then set the dates for the Sailing period (3.).
Leave the Default value (4.) at 0% and press Apply (5.) when you are done.
Next step is to switch to Advanced (1.) tab and check the Excludes other discounts (2.) and press Apply (3.).
This will set that no other discounts will be valid in the selected period.