Hi Annette: I had a heck of a time with hornets this year. I tried the idea of poisoning the hornets using cat food with dog tick medicine. (Making sure the poison was out of reach by cats, dogs, etc.). That worked. The hornets really liked the cat food. I tried traps and that worked somewhat. In fact the traps would fill up with hornets in a very short period of time. (Perhaps, I needed more traps. I had only three.) Still the hornets kept coming back in large numbers. So, for the next few days, I stood by the hive for about 20 min. at a time, with a fly swatter. I would do this 3 times a day. I ended up killing alot of hornets. To bring the hornets into the area of the hive quicker, I would partially remove the top cover. The smell from the open hive seem to attract them quickly.
Eventually, I got the situation under control. The hive appears to be able to drive out the 1 or 2 hornets I've seen trying to enter the hive. All of the ideas above worked, however the sheer number of hornets made controlling them a problem. In eastern Washington this year, hornets/yellow jackets have been a real problem.
Good luck with yellow jackets.