What strikes me in this discussion is people saying to be afraid to leave a thruthfull review, as a horrible guest could come back and get them.
Principally, it's not a good idea to take locals or family members of locals. I do that only, if they announce a good reason with the booking.
Someone from another town or country is less likely to come back.
I wonder how you can host, if you fear your guests, whilst they are here or even later. After a while, you have a dozen potential revenge seakers out there. That must be a very scary way of life.
On the other hand, accepting airbnb's offer, profiting from the better evaluation through peer teview, means also an obligation to the community.
If you do nothing, if you encounter a dangerous guest, you allow him to go on. If you call the help line to discuss concerns about a strange booking, they can look up the history and may tell you, that the guest made a few more trips with no incident - that should be reassuring. It should not mean: no incidents or hosts scared witless.
You should at least write a review, give the thumbs down sign and write half a line very neutral text. That way he cannot instant book with hosts asking for reviews, you wrote nothing offensive and another host can imagine that there was nothing positive to say.
In the airbnb feedback, you should write what scared you. The guest sees neither the thumbs dosn nor the airbnb feedback. If another host has a real problem and calls the gelpline, they will see those remarks It may lend credibility to a fellow hosts claim.
I give the thimbs down rarely, after asking myself, if the behaviour provoked only me or will be horrible for most hosts.
If someone is so dangerous, that revenge seems probable, I call, flag the profile, write feedback. I see that as parr of my obligations in using a service like airbnb in contrast to a simple ad on Craigslist.
(Sorry for the spelling, I can't edit on the phone)