Great question Steve! You are able to control this by navigating to Settings>Availability & Calendar>General Availability

*Note: Make sure you've also enabled online booking under Settings>Booking Options so clients can book with you

On this screen, you can set your availability for each day of the week. In addition, once a client books with you, that time will be automatically blocked and hidden from your online calendar for others to book. Note that if you have multiple staff members, it will only block the calendar of the staff member they booked with. 

To learn more about setting your general availability, take a look at this article. Thanks for your post! If you have any other questions or run into trouble, leave a comment below!