@Linkat,
There's one trick which I've had complete success with but it takes more nerve than some people have...
You buy a big can of wasp/hornet spray at Home Depot (step one)...
Now, the last thing you want to do is tip-toe up to the thing, one or two quick zaps with the can of spray, and then run, you'll be lucky if you live.
The thing you want to do is pick the coolest time of ay or night, make sure you've got some sort of a light on the thing (person two), walk up to about six feet away from it, and just stand there and empty the entire can on the thing.
Within my experience, the hornets or wasps will just keep on trying to fly back INTO the thing and they'll all die. A hornet/wasp nest is basically made of paper and at least the fumes go all the way through it.