You can activate in the configuration Scheduling page > Settings > Advanced settings > Customer login required so that only customers or patients you have accepted are allowed to book with you. This can be useful if, for example, you already have a full customer/patient base and only want to take on limited or no customers.
You can either import existing customers into eTermin so that they can book appointments from this point in time. Or you can send everyone a link to book an appointment so that everyone has to register once to book. The latter has the advantage that, should it have changed in the meantime, the customer data can be updated automatically when your customer books their appointment.