Block Client

[sensational]

Is there a way to block a client so they do not have the ability to book online?

[JJ_]

There's not currently a way to blacklist or block a specific client from booking. 

You can maybe try using the No-Show Protection feature and require a card on file for booking.

If your client doesn’t show up or cancels outside of your cancellation window, you can choose to enforce your cancellation policy and charge their card on file. 

Check out this article and hopefully, this works for you:

Set a Custom Cancellation Policy with Square Appointments

[Ty863]

There needs to bea way to block/ban a bad client from making an appointment