Where possible, speak to the person face-to-face. If others are around, try and step aside so as to give them your undivided attention and so as not to widen the issue. Make sure they understand that you are fully focused on their concern. People want to be heard, taken seriously and listened to.
Sounds simple, but too often, due to time pressures complaints are not recorded. Do it! Why? Because you need to measure the rate and content of complaints so you can continue to improve the quality of your event.