you should be gathering eggs everyday to prevent broodiness and any chance of egg eating. CMV and lauralou are right, wire bottom cages are the best. If you dont have one, seperate her anywhere you can and dont give her a nest. And if you dont have anywhere to seperate her, make sure you collect all the eggs daily, twice daily if your hens lay at different time.
Also, push her off the nest. If she's stuck on one specific box, close it off, take out the bedding, etc. Unless she's super determined she wont like the idea of you coming along and pulling her off her nest and will eventually give up if she thinks the nest isn't a 'safe' place anymore.