You may want only some services to be available to past clients. For example, you may want consultations to be only booked by new clients or lash fills and touch-ups to be only booked by current clients.
You can edit the booking preferences when creating or editing your services. For each one, decide if you want the service available to all clients, current clients only, or new clients only.
Open up the service you want to be only available to certain clients
Under "Client availability" select "Current clients only"
When this is selected, only clients which have been added to your client files (either by you, or by having booked before) can book that type of service.