You can blacklist a guest by ticking the “blacklist” box when in Manage Bookings.
The email address(es) and phone(s) in the booking you thus mark are used when a new booking is made. If there is any match, the new booking will be waitlisted without any indication to the guest of any reason. When you open the booking as admin in Manage Bookings, a message at the bottom of the screen will list the blacklisted booking(s) which match either a phone number or an email address in the booking you are looking at.
Note that false positives are possible. Phone numbers are compared based on the last 8 digits, because area/country codes are variable.
The feature won’t act until you mark one or more bookings as blacklisted.
If more than a few clubs start using this feature we will extend the matching to be across all clubs, so the blacklist will be more powerful. At that point the blacklist may affect you even if you haven’t marked any bookings.