I hate to sound like a curmudgeon here, but...why do you need the nest? I mean, it belongs to those hornets and it would appear they're still using it. If they're not hurting you, leave them be. If they're around the house stinging your kids, that's one thing. But you said they're out in Nature where they're not hurting anyone. I'm not trying to criticize you here. I'm just asking you to consider if having their home is worth killing all those creatures who are doing you no harm. I understand that in winter they abandon the nest. So, wouldn't that be better?