How to Block a Customer
To block a customer search for the customer in your magic search in the top right of your diary:

Select the customers profile to open the profile page, scroll down to the bottom and click block:

How to Unblock a Customer
There are two ways to unblock a customer. You can always see their history by searching for them in the Search field and looking at their history.
Go to Reports - Customer List - untick search by visit date - press get list - press unblock against the customer.
When making a booking and entering the customer's details it will prompt you if they are blocked

If a customer tries to book online and their profile is blocked they will see the below message:

