Luckily none of my birds have been broody yet. the chicks are too young, and the isa browns have had broodiness practically bred out of them.
I have read though, that to break broodyness, you must keep the hen away from the nesting box. Pop her in a cat crate or wire cage without bedding (but with food and water obviously!) for a couple of days then let her back out, if she heads straight back to the nest box, back into the crate, until the broodiness is gone.
Depending on the bird, broodiness can be broken in a couple of days, or if you're unlucky (or the hen is persistent) broodiness can go on for weeks