The funny thing about hens, and also other birds like parakeets and parrots, is that they don't know their eggs will never hatch without a mate. When the broodiness hits them, the hormones take over and they become obsessed with setting on those eggs. It can be very hard to break the broody behavior. I never really try. I lift up my broody and take away the eggs daily, and also I take them off the nest and put them outside every chance I get. They do lose weight and sometimes they pluck off all their breast/belly feathers to make the nest. Those feathers will grow back.
You can also put them in a wire cage with no bedding of any sort--just food and water. I've heard that this will break them. Make sure, if you do this, that you put the cage in out in the open so that it's not real nice and cozy.
You could also just get a few fertile eggs from a chicken friend and stick those under her....
