From Feathersite:
"Breaking Up" a Broody Hen
We don't always want to have our hens hatching eggs. When we want to stop one, this is called "breaking up" a broody. Sometimes just putting her in a pen where she can't see her old nest and keeping her there for 4 days will do the job. She should, of course, have feed and water. Some strong broodies will just continue to set even in a pen with no eggs. For the more stubborn hen, a wire-bottomed cage is necessary. The airflow up through the wire keeps her underside cool and after a few days she will usually give up. Again, she should have feed and water available at all times. Some commercial people and old-time chicken raisers deprive a hen of feed and water when trying to break her up, but this is cruel and also not good for the bird. Lack of feed weakens an already weak bird (since they don't eat much when broody anyway) and lack of water for several days can damage the liver. I just learned of another trick. Put a 'clutch" of ice cubes under her. Sometimes it'll take 2 "clutches" but it should cure her.
Some hens (especially Cochin bantams and Silkies) just don't want to quit. Another trick I've found that helps with these is to put them in a pen with nothing but feed, water and a very active young cockerel. He just won't let them sit there as he'll constantly be trying to mate them.