If they try to book, change the rate to $9000/day for them.
They have to approve the change or they can't book, and you don't get dinged for canceling them.
Otherwise, not aware of a way to "block" a potential renter other than marking their request as "Spam". Then you never get any more Inquiries or Booking Requests from that user.
But you can't do any of this in advance.