I need my chickens to stop laying

I was devastated to learn that my sick chicken is showing symptoms of lead poisoning and probably wont recover. My other 3 gorgeous girls look healthy and happy, however being in the same pen, have most likely also been exposed. Investigation into their pen resulted in finding tiny paint flecks from the original house paint (containing lead ) Once it is in their system it is passed through to their eggs which can then make us sick.

A massive clean up planned for the weekend will involve removal of approximately 6 -10 inches off the surface of their enclosure. Broken slabs of concrete placed down, so that they cannot reach the contaminated soil and clean topsoil on top. Once I am confident that the pen is "lead proof" I will invest in some new chickens, however I love my girls soo much I can't bring myself to cull my 3 loving loyal gardeners and as we have plenty of room for new and old, am quite prepared to keep Diamond, liquorice and Lemonade provided I can be sure they wont lay any more eggs that could get mixed up with the new layers. They are Red Shavers (bred to lay)

So my question is.... What can I do to stop them laying permanently?? Without causing anymore health problems and costing a fortune?? Then they can live out their days doing as they always do, chatting to me and following me around while I carry on with my jobs