You can try several things.
1. Sometimes all it takes is getting the hen off the nest a few times - plop her down as far from the nest as you can, if you free range take her somewhere out of sight if possible. This sometimes can work if you catch it quick enough I have found.
2. Or you can use a wire dog crate as a broody busting box. It should be lifted off the ground on blocks, you want it uncomfortable to lay down. And in a bright spot (but not in direct sun in hot weather)
Usually it just takes me a few days with these methods. Some birds might become repeat offenders though. Those I will usually let sit on a nest and just go ahead and let them hatch!