You can set up all of your availability ahead of time, using our monthly calendar availability. However, you can also create a once-off unavailable block.
Go to the Calendar, then click on the starting time cell that you'd like to create your unavailable block at, and select Busy time.
Confirm the start and end time and add a note if necessary, which will appear on your calendar.
You can also repeat an unavailable block, the same as you would when creating an appointment.