Hi Zombee (like it!) and welcome to the Forum.
Bad luck, happens to most of us sooner or later. The issue is how far they have got a hold so far.
The easiest course is prevention and that means small entrances all year round. The standard full entrance on a National is way more than most full colonies would either choose or need and leaves the guard bees an almost impossible job to defend. it probably dates from a time when most colonies were on solid floors and was to provide "ventilation" - in practice I suspect it was not necessary then either, Most of my floors have built in entrances only maybe 3cm wide and 1cm deep all year. It means the entrances are easily defended and if there is a slight crowd at the entrance sometimes, all the better to deter robbing bees and chancer wasps. You can use pieces of cut down frame or scrap timber to block most of it off - I have temporary fixes which have been in place years!
If they have already got in and are being ignored by guard bees then it is a lot trickier because the guard bees have probably been killed or given up and the wasps have acquired some colony scent and are coming or going as they please. I would still restrict the entrance to a single bee space the size of a pencil maybe to really make incursions difficult and if they are still going in and out, then I have heard that a sheet of glass leant against the front of the hive or a short tunnel fitted can baffle the wasps but the bees work out. Or you may be able to temporarily house the hive over 3 miles away until the local wasps have subsided - but do put preventative measures on the second hive, in case they just switch to the next victim.
Truth is poorly defended small hives and nucs are particularly vulnerable and a large healthy colony with a realistically sized entrance should not usually have a problem. Feral colonies often naturally close up excessively large entrances with propolis (apparently the word propolis comes from the Greek "walls of the city"), though I only had it significantly happen once on a hive, before I deliberately closed them all right down.
This is probably controversial but I have never used wasp traps against common wasps and since fitting small entrances all year round have not lost a colony to wasps so have not been tempted to, even though most of my colonies are in orchards full of wasps. I don't think they work. Wasps have their role and trapping them in their thousands, when that is not the answer, is just be punitive and the significant by-catch of other insects unjustifiable.
The likely arrival of Asian Hornet may require me to change my take on it.
Other views are available. Best of luck and let us know how you get on, you will not be alone.
