We use tags... In our case we call them "Problem Guest" and it can be done at any point...we then make notes about why we have tagged them. Then we have a trigger that we use that sends us an email at the time a booking is created where the tag of Problem Guest is attached to the guest. The real question is what/how you want to address the situation when you get one of those emails.
The email template system is not "smart enough" to be able to give you the guest history information or "notes" gathered from previous bookings or quotes for the guest (at least not that we have found yet) so we will go and lookup the current booking and will then review the history of the guest and determine why we have flagged them and make the determination of whether we call them and ultimately cancel them.
It happens very rarely for us, but we still like to know and make the appropriate determination.
