Contagiouse Brooding!

So, our one australorp went broody about a week ago now, and then one of our ee's went broody and now my cochin is in the very last nesting box tonight and wont leave... 15 laying hens only use three out of the 6 nesting boxes we offer them, and if we have three broody hens plugging them up where will the other ladies lay their eggs! We also don't want a vicious cycle of brooding! Heaven help us if all 15 chickens decide to go broody and start fighting over three nesting boxes! It'll be a chicken world war!

Is there a way to break the chain?