To stop an appointment slot from being booked you can block it from being available through the editor.
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First, you must open the appointment in the editor:

Click on the appointment you want to change

Next, click on the block button for the time slot you which to block:

Click on Yes option

Scroll up and click on the Save button to save changes

Then, in the store you will see that the option to select that time slot is now gone:

You can also unblock the time slot and it will appear back onto the store:

Click on Yes option

Click Save button

Then, in the store you will see that the option to select that time slot is available again :