At the recent Independent Hotel 12 TripAdvisor session many of the questions from hoteliers related to an increasing and worrying phenomenon - the guest requesting a discount "because you wouldn't want me to post a 'one star' review."
Alistair from TripAdvisor found this a difficult one to answer, because, as one hotelier pointed out: "You don't insist on verification [real name/date of stay] so we can't positively identify the 'blackmailer'."
We have encountered this with clients, and we have thought long and hard about the best way to deal with this that doesn't involve TripAdvisor. Our conclusion: let guests know that this kind of thing simply won't be countenanced. Nicely!
By displaying a notice something like this:
You will, of course, decide on your own wording, but the final statement that effectively says "Don't wait until check-out to complain - and don't expect a discount." is, we think, the key.