Ok, you need to turf her out the nest box. As soon as the others have finished laying you need to close the coop up to stop her going in. others recommend dipping her bottom in cold water, not something i would try, it dosent sound too humane to me, but i am a softie
she possibly wont lay any eggs for maybe 6 weeks though so dont worry if you dont get any.Letting her sit will only encourage her broodiness and i dont think its fair to let her sit without having any chicks at the end of it so be firm anf keep turfing her out