Blacklist

Although I agree with this feature I should say it needs to be something the staff members can initiate. I mean as an admin or other more privileged role in wordpress I can "block the user" to some extent.

https://www.wpbeginner.com/plugins/how-to-block-a-wordpress-user-without-deleting-their-account/

Perhaps just creating a new role for "blocked users" and then making it so when they log in they are redirected to a page with a message explaining. I imagine this is possible with some basic plugins outside of BookingPress. So you would just need to change the customer's role to accomplish this. However, that blocks them from booking with anyone and maybe you just want to block them from a particular staff member.

Further I should say it would be better if there were a way for a staff member to initiate the "block" of the customer. They should not need someone with more elevated permissions to get involved in my opinion.