As others have said- fencing OR motion activated sprinklers. I have one for my big garden in the back that the girls don't bother going into until the fall (not sure why....) and it works great to keep out the wild turkeys.
I was watering my flower garden the other day on an rotating sprinkler head and I can attest to the fact that my hen ran like the dickens when it followed her around the garden by chance, so I have every reason to believe the motion activated sprinkler would work great as well. They cover a nice big area too. Way cheaper than fencing if you have a large area.