To break the broody urge, you need to keep cool air circulating under her. She craves a warm, dark, snug place and will then go into a trance once she's found it. For a month she won't want to eat or drink, and being around her flock mates will infuriate her - she just wants to be alone!
What I do when I see the first sign of a hen going broody, even before she plants herself on a nest, when she starts that incessant, low, broody clucking, I put her into an open-mesh bottomed cage inside the run if it's daytime, and inside the garage at night with a fan blowing under her during the night. I let her out every couple hours during the day to eat and drink, but without access to the coop. This prevents her from losing her rank in the pecking order, and counters the urge to be alone.
Usually, with this method, I can return a broody to normal life within two days, and two nights.