I own a very successful clinical massage studio. Occasionally clients will come in, have a great session, leave very grateful and smiling (having given a nice gratuity), and in the next day or two, call back and say they were not happy with their session and ask for a free one or a refund. It's so very rare that clients are not happy. When some clients aren't, they typically immediately address it with the massage therapist or the receptionist directly after their appointment. We always provide discounts and follow-up on people who are unsatisfied (one even wrote a review saying they appreciated the free session and that they were happy we made it right).
My question for you is, how can we dissuade would-be scammers from trying to get free work? It's so rare that anyone is unhappy, and it's hard not feel exploited when people who seemed thrilled with their session call days later asking for a free one, claiming to be unhappy.
Etiquetteer sympathizes with your feelings of exploitation. This condition is not limited to your situation, alas. Etiquetteer has long said that you really know how successful your party was based on what the guests say three weeks later; it's not always what they said when they left. Your query even reminded Etiquetteer of a couple who were banned for life by a cruise line because of the number and volume of their complaints. But that seems rather extreme for your studio, especially as you say it's a rare occurrence.
You and your colleagues have the opportunity to let clients know that their feedback is both important, and will be taken at their word. Does your massage studio use one of those intake forms that some other studios use? You might add to it a checkbox with something along the lines of "Your honest and candid feedback about your massage experience will help us serve you better at future appointments. As the client, it's your responsibility to bring to our attention any problems or concerns during and immediately after your massage." It could also be a placard in each massage room.
Perhaps you could invite them to record their comments in a guest book on site, before they leave? Another possibility: next-day follow-up with clients you find suspiciously satisfied to see how they feel - and perhaps to schedule another appointment. Use their feedback in your message.
Etiquetteer wishes you and your colleagues continued success in practicing your healing arts on appreciative and grateful clients.