"You can't "blacklist" somebody for having a bad day. It they were doing it out of mischief then yes, but not if they're not well enough to fly. Maybe this pax just needed a few days to get herself sorted out prior to travelling, not just one?"
Oh I disagree, if someone came to your house party and had a nervous evening and generally caused a downer for your other 00's of guests, you'd not invite him or her back. Airlines should be perfectly in their right to do the same thing.