You can try to pick off all the balls as they grow, were you using fem or bagseed, and check your grow area for light leaks. Stress is the number one cause for normal seeds to hermie, limiting stress is your number one concern when it comes to keeping a plant from going hermie on ya. If you grew from feminized seeds then they could have possibly had a genetic predisposition to hermie, and if you grew from bagseed then the plant that pollinated the bud was probably a hermie.