Can I deactivate client's access to my Online Booking?

You can close your calendar in your Online Portal. This will allow you to close your calendar until you are ready for clients to start booking with you, and allow you to book appointments in the Online Calendar on behalf of your clients. Since your availability will be closed, you will have to book your appointments in Override settings. 

Here's a Help Guide: How to close your Booking Calendar

Here's a Help Guide: How to Book an Override Appointment