shell-less eggs

Hi,
One of my older girls stopped laying in April this year after laying 2/3 soft eggs. She did not look too good for a few weeks with her comb getting quite dark. To be perfectly honest I thought that she was just going to fade away because of a bad heart of something age related.
Well in the last few weeks she seems to have perked up no end. Since we've had new pullets she has been looking good with a lovely red comb. I think it has done her good to not be at the bottom of the pecking order.
The thing is; she layed an egg last friday to everyone's surprise (including hers).
Since then she has layed another soft egg. It must be age related because she eats plenty of layers pellets and has access to oyster shell as they all do. But I am worried that she might get an egg stuck inside her due to them being soft.
My question is: How do I encourage her to stop laying for her own sake and just have her healthy and happy to do nothing.

as magicaltoad has said you can't stop from ever laying again, however you could try boosting her calcium by giving her a top up of limestone powder for about 3 or 4 days. Just give her a pinch on something like half a grape or a couple of sweetcorn kernals 

Thank you all for your replies. I did suspect that I could not stop her from laying.
I will get some limestone powder for her while I'm off work tomorrow.
Cheers.