mine do the same thing.. i have 15 hens and they all have their favorite spot in the coop....i just let them alone.. they only use 3 out of the 8 nest boxes anyway. most love to roost on top of the nest boxes.. i used to keep a heat lamp up there in the winter but the chickens were getting under it somehow and burning their feathers off their backs...
so i don't do that anymore.. they are fine anyway.. maybe you could place blocks in the nest box and remove them every morning!!!