Honestly I don't know that there's any guaranteed way to prevent laying. They don't all do it, I've had a 'tiel hen who laid maybe 8 or 9 eggs in her whole 17 year lifetime with no rooster around at all, and another who laid no eggs even though there was a VERY amorous rooster around her! You can reduce the chances of it happening by only petting her on the head and neck, not making any nestbox available and prohibiting access to dark "nesty" type spaces in the home, and a low fat pelleted diet with lots of fresh low-calorie vegetables may help too - birds on a high fat diet can perceive that it's a good time to lay because of plentiful food. But even so, some of them will lay eggs on the bare floor of a cage with no encouragement (like mine!) and others just never do, just depending on their chemical/hormonal make-up. So it's a good idea to have those dummy eggs on standby, just in case