You can close single days or only a specific departure for your activities.
To close days start by opening the product you need to close. "Products -> Tours& Activties", select the product from the list to open it.
Select the availability tab in the booking.
There you have a calendar showing you a week overview of your availability.
10/10 means that you have 10 out of 10 seat still available for sale. 8/10 means you have 8 out of 10 still available.
x - select the x to delete availability. Deleting availability means that the day will appear as white and not bookable in your online booking engine.
To close a departure select the checkbox by close on the departure.
To close a day, select the closed checkbox at the bottom of the page.
When you close a day, it will appear red and un-bookable in your online booking engine.
Here you can find information on how to close longer data ranges in bulk, not just day by day.