No personal experience with killing hornets, but with wasps and bees, I'd put emphasis on the run like heck though. You might check with extensions and universities and on the web. Do a search on hornets. I understand that hornets build an round/oval shaped nest from mud, twigs, and paper, etc. They may be flying out of a crack but their big nest may be hanging in the attic where it won't get hit by the spray.
I've heard of sneaking up on a nest at night a sticking a bag over the nest but I wouldn't do it. Contact a beekeeper who has experience with these matters. It may be bees instead of hornets anyway. Bees aren't as mean as hornets who are prone to mass stinging.