If she refuses to give up you could put her in a wire bottom cage to cool off her chest and break her broodiness. I don't have such cage so was thinking of sneaking her a chick at night and hoping for the best. If doesn't work will have to keep getting her out of coop and hope she gives up.
I had two hatch from a broody a week late and both are doing fine. I do have two more that are over that and the water test was positive, but with no movement. Ready to get rid of them, but poor hen is still sitting.