You may place a guest as blacklisted by going to Customers Tab> Guests>Search for guest name>Guest Details> Select the below icon from the drop-down list>You may also place a message on the notes stating why the guest is blacklisted>Save Changes
If you create a booking under the same name, this will appear on on the reservations tab which will alert you that the guest is blaclisted.
Please note that placing a guest on blacklist in GuestPoint will not stop the guest from booking through Expedia, Booking.com and other online travel agents.
If guest was able to book through OTAs, it is still up to your discretion if you will accommodate the guest. If you chose to decline the guest, you may inform the travel agent to cancel the booking as guest is blacklisted.