Watch the bushes from a distance at night. Hornets, as with other wasps, settle down at night. You can watch as they come in for the evening and determine the exact location of the nests.
Spray around the bushes with an insecticide that kills hornets to catch any insects that are late to arrive for the night, and to kill those that manage to escape the nest. Spray an area several feet in all directions around the bushes.
Put a straw nozzle on the spray can and carefully insert it into the bush near the nest. Spray the nest quickly with the poison and walk away. Some of the hornets will make it out of the nest, and you do not want to be nearby. Wait for the hornets to die down, and go back out to spray the next nest. If they do not calm down after a half-hour or so, wait and do the next nest the following evening.
Wait two or three days after you have sprayed all of the nests, and then sit out in the evening again and watch for hornets to come to the nests. If you see no activity, then you have succeeded. If a few hornets come back to the nest, repeat the process another evening to finish off the insects.