My secret weapons are these:
My plan was to keep the offending hen away from the coop & nest boxes at all costs. Exhibit A is for daytime, Exhibit B for night time in the house. Their annoyance at being contained in such a manner ensures broodiness is quite quickly forgotten. Four days in the anti-broody pens seems to do the trick.
I'm very pleased not to have to suffer four weeks plus of a grumpy nest-hogger! :D