You can block a guest from booking for future events by blocking either their email address of their mobile number if you are using Mobile Verify.
Pro tip: Blocking an email address will help stop a guest from booking for your other events but it is easy to obtain multiple email addresses. Mobile Verify is the highest form of security you can use as it is much more difficult to have multiple mobile numbers.
You can block a guest on the Manage Guest screen. Just find their order and in the Order Actions section choose the Block option. In the pop-up you can choose to block their email address and their mobile phone number if you have Mobile Verify enabled for the event.
Once blocked you can view all the email addresses and mobile numbers you have blocked
here