Hornets' nest

[NUTTSGT]

If it was warmer out, I recommend Sevin dust to kill them.

Duct tape a section of brake line on to an air nozzle. Fill the brake line up with sevin dust, attach to the air hose. Stick the end of the brake line into the nest hole and give it a squeeze. They'll be dead by the next day.

 

[KellyfromVA]

If you don't have any hornet or wasp killing spray handy, I've found PB Blaster kills them just as quickly. Wait till night, then spray into and onto the nest. Between the fumes and soaking the paper walls with the stuff, they'll be dead in seconds.