Customer Portal
To disable a customer's access to the portal, you can do so by changing or renaming the customer's email address. Since the email address is used as the login username, modifying it will prevent the customer from logging in or receiving a password reset via email.
In this video, you can see how to modify the customer's email and the resulting experience for the customer when they try to log in or reset their password.
Online Booking
This is slightly more complex as we have 3 methods you can set on how the customers can create a booking with you.
Allow Guest check out > This enables any customer to create a booking
Allow Log in > This allows customers with the password created to log in
Allow Customers Sign Up > This allows any customers who do not have their email yet in your account to sign up and create an online booking
With the information above, the only way to prevent customers from booking online is to require all customers to sign in, which prevents any guests or customers from signing up.
You can then rename the customer's email as above to restrict access to the Customer Portal.