Dunking the bird's undercarriage in cold water can cure broodiness in some hens - but a lot of people think it's "cruel".  Maybe letting them starve and dehydrate because they're utterly broody and won't leave the nest box is less cruel?  Hmmmm . . . .

A broody hen's temperature is about 2 degrees hotter than normal, I believe, so the object is to cool her down.  I use an old dog cage, mesh on all sides, floor and top, and I don't try to protect them from the elements unless it's heavy rain.  You don't allow them any nesting material and you try to support the cage off the ground so that the wind gets under her tail!  You'll soon learn how a broody acts - strange quiet clucking to herself, sitting tight on the nest, getting aggressive when you grab her to move her (it's sensible to grab them from behind if they try to peck you - a peck on the back of the hand from a big bird can draw blood) but each hen seems to be different in the amount of time they need in the "cooler"; I've had some in the cage for a week before they stop running back to the nest box as soon as you release them.  The good news is that once you've stopped them being broody they very soon get back into lay.