You may have a specific session that is only available for a specific amount of time. This could be due to:
Discounted rate
Full-term booking
You do not want to take any further bookings for this session
How to make this change within your activity settings
Select Edit Activity
Scroll down to Sessions and Costs
Click on the padlock beside the session you wish to close
Save changes
What this will look like on the booking form
The session will appear as crossed out and will not be bookable at all by your customers: