My hens too are emptying out the nest boxes. At least I think it's them. The only other culprit would be the cockeral that the girls hatched back on July 4th.
I've put hay in twice only to have it kicked out or tossed into nothing that resembles a nest. I give up. They're all molting anyway and not laying. I won't be putting any back in until the spring or maybe not until I see an egg!
Edited: But wait...yours are pullets. So they'll maybe be laying soon. What you can do is put the hay in, form a bowl nest with your hands and put a golf ball or two in the nest. My pullets never dropped an egg out of the box when I use this method, all first eggs were laid in the nest, and yes, they do explore up until that point and will kick out the hay until they get the urge to lay their first egg. Then hopefully they'll stop that until molting season or after they've hatched a little stinker boy roo.