Just to clarify what John said. The image data in your NEF file is never altered by ACR. All of the adjustments that you make to the file are saved as metadata. If you mean that you don't want the NEF file to reflect those changes after you save the JPEG image, just click on the Cancel button rather than the Done button after you have saved your JPEG image. If you do that, then ACR will not even write the changes to the metadata of your NEF file.
But even if you have clicked on the Done button and saved those settings, ypu can always clear the settings and restore the NEF file to its original state.