I did some more research and have found the "water-flooding" suggestion mentioned in several spots. I also stumbled across a suggestion from a different university to mulch deeply (3" minimum) as this is suppose to dissuade the female wasps from calling your garden their home. A third site run by a college professor encourages plugging the entrance with a stick and leveling the soil around it every morning for a few days. The theory is that the Cicada Killer Wasps use visual clues to locate their nest and, by re-distributing the soil, they cannot find their way back easily. In addition, other female Cicada Wasps are attracted to areas where nesting is already occurring and, so, they will no longer hone in on your garden if they don't see the burrowing activity.
As I don't have any first hand experience, though, I do not know how effective any of these treatments really are. Please let us know if you find something that works.
