I'm assuming that you have separate cages for your birds - if not you will have to make arrangements to house them separately from here on in. I had a male and female cockatiel years ago and there is no way I ever could have had them in the same cage, otherwise I'd have been up to my neck in baby birds! They had their own separate cages which were placed side by side - of course they had LOTS of out-of-cage time to be together but it was strictly supervised to ensure no mating activity took place. Even so, sometimes with very chronic egg laying hens this is not enough, they can and do often continue to lay eggs even if they have not mated, that urge to reproduce is so very strong in some and it can be very difficult to persuade them otherwise!
If you have not already done so, I would urge you to invest in some dummy cockatiel eggs. Bear in mind that as she's laid one, there may well already be another on the way, but giving her fake eggs to sit on might help to convince her that she's already laid a clutch and doesn't need to produce any more. Not guaranteed, but it may help.
It might also be time for you to have her assessed by a certified avian veterinarian who may be able to recommend some management practices for you, and even some hormonal treatment for your hen if it seems that the egg laying has the potential to become chronic. It's very important to consult an avian specialist in a case like this as it's not something that should be handled by a vet who mainly sees dogs, cats or other mammalian type patients. I'm not sure exactly where in the world you are located, but the following list may hopefully help you find an avian specialist near you if you are not already familiar with one ...
After a suggestion by a member, @Abu Colo, the Mod Team decided to create a list of veterinarians around the world who see birds. This was something we had discussed a few years ago, but the task seemed so large that we didn't get very far. However, the need is there and we decided that the time...
www.parrotforums.com
I wish you the very best of luck with your wayward little hen!