I recall doing some trail work in upstate South Carolina in 2001. They had just had a quite warm winter, and there was a rabies outbreak ravaging the skunk population. Cold winters and skunk predation were the two major controls on the hornet population, so without those controls, that year was MISERABLE for hornets. I was stung time after time, and it seemed that we hit another nest about every 20 feet as we blazed new trail.
This is not to suggest importing skunks. But maybe there is other biological control that can help you- a parasitic wasp or mite?
Paul has a recent Youtube
video about beneficial effects of wasps/ not sure if that would tie in to your hornet situation.