When a chicken goes broody her body temperature increases by a few degrees and she will stay broody for as long as her temperature is raised. In order to 'break' her broodiness you need to cool her body down to normal.
There are a few things you can try:
Try dunking her belly in a bucket of cold water for a minute or so, then put her down. If she heads back towards her nest, do it again, and keep doing It until she stops. This has worked for me but generally only works early on in her broodiness.
Another thing to do is place her in a wire cage a few feet off the ground with no nesting material. Make sure there is airflow in the cage, as this will decrease her temperature. Make sure she has food and water. She will needs to be in there a few days.
You could try and completely block her off from her nest, this has worked for me before, but it was in the first hour or so of broodiness.
Whatever you do, make sure you do it as soon as possible, as the longer she is broody, the harder it will ecome to break her.
Hope this helps!!