Hiya,
I have just read this thread.
Firstly - sorry to hear of your situation.
Secondly - if you have never killed a duck before, DO NOT ATTEMPT IT! They have very strong necks!
They need to be electrically stunned then have both carotid arteries cut and bled, any other way and they will suffer.
If you really do want to kill them may I suggest you call Bristol University who have an onsite poultry abattoir and are all very experienced in the humane slaughter of poultry.
You will not get much meat off a Campbell duck. Have you thought about having them euthanized by the vet?
I would suggest if you really have had no luck at all rehoming them then have them euthanized. How far are you willing to travel to rehome them? I am in Dorset and I rescue ducks, I would be happy to take them, could meet you half way if you like?
I have 10 pet ducks myself, so don't have room for any more, but I have a farm where I can take them where they can live semi free with a large pond.
I must stress do not try and kill them unless you are experienced, ducks are much harder to kill than chickens!
Please feel free to private message me on this.