Well, I HAVE tried the ice cube thing and trust me, it doesn't work. Broody hens will be content to sit on anything. And I've tried the dunking in cold water trick, too. Nope. Doesn't work either.
What does work is to place the broody hen in an open-mesh bottom cage with a fan blowing at an angle under her. I currently have my third broody this week in the cage out in the garage. Usually, this method will cure a broody in three days and two nights. You need to keep this up around the clock.
The final test to see if she's broody-broke is to turn her loose and see if she heads directly for a nest box. If she does, she needs another day in the cage.
By the way, I was too busy with the first broody to go to the trouble of setting up the fan, and she stayed stubbornly broody for nine days until I finally broke down and got the fan out. It took just one night with the fan blowing on her underparts to end her broody episode. So the fan is absolutely essential if you want to get this over with fast.