I’d like to see a feature to allow banning certain (rude/inappropriate) customers from rebooking.
Are you using our Bookings API to book customers? If so you could block specific customers from booking by the customer_id. Otherwise with our 1st party offering we don’t have a way to block customers from booking. We’re constantly working to improve our features based on feedback like this, so I’ll be sure to share your request to the API product team. 
I’m not using Bookings API, I was hoping it could be added to Square’s native UI, since it doesn’t sound too complicate to implement based on your answer. Please forward my feedback, I’ll be happy to see it becomes available in the future. Thanks.