Paul, a wasp trap might reduce their numbers a bit. If you do a search of the forum about yellow jacket traps you will find that quite a few people have success using a suspended plastic bottle with some coke/pepsi, some vinegar to keep the bees out , and a saveloy in it. There's a few variations on the quantities of the ingredients, but they definately love a bit of meat - saveloy, mince, wallaby legs... so with a suitable wasp poison or drowning agent added, you can get rid of a lot of wasps. Some nice condiment added to the meat like the old derris dust, they might even take home with them to share with their mates. One of the artificial sweeteners is pretty deadly on them too, Sevin I think, but someone here will know which one it is.
I have also seen artificial wasp nests that you can buy to place in your apiary to deter wasps from other nests from hanging around. I don't know if that works, but could be worth a try. If you find their nest, some two stroke fuel added after dark makes them pretty ill and I have heard that a clear plastic bottle jammed in their hive/burrow entrance will kill them too as if there is light still coming in, they fail to realise that they need to dig a new entrance before their air runs out.