Customers complain for a number of reasons; you did something wrong, they had a bad day, they are critics or they are after a freebie. If you did something wrong, fix it. The middle two can benefit from a little tender care. The latter is a danger that needs spotting early. Basically, the professional complainer is after a comp of some kind. There is often a threat made to your reputation or a guarantee they are taking liberties over.

The classic scenario will work like this:

They arrive and start complaining almost straight away.
The food will be finished off, but they will complain about the standard, often very harshly.
Usually the complaint will come after their last course, again with clean plates.
Drinks will be kept to a minimum, almost to the point of looking cheap.
There will be some very noise to speak to a manager
Then when they see the manager, they will ask to talk in private.

Once you are in private there will be open threats to your reputation and they will be looking for you to crumble and make a full comp. Often this is given, sometimes for food only, which is why the minimum drinks stick out.

If a booking starts to look like the above and the rest of the room is having a good time, then there is a high probability that you have a professional trying to sting you. If the food has been finished without any complaint during the course, why was this not raised before? Does the timing seem out of character with what they are saying?

Any such complaints should be handled in public and be prepared for any adverse reviews that may come your way. It is a very basic form of blackmail, most of the time there will be no comebacks if you stand your ground. Keep checking the web and local papers for any reviews. If something does come up, ask the site or editor to recant the article and submit a statement on what happened to back this up.

Genuine complaints can be handled in a genuine fashion. Professional complainers are out to take advantage of any site that shows a weakness and they will pass this on to others. This may sound harsh but if you operate a quality venue you will know what a genuine complaint looks like. One very effective countermeasure is to share information with other local operators, it’s a two way flow.