Certified GKH Gardening Expert
About the only thing I can think of is spot treating it with Round-up. While this will also kill the lawn grass, spraying only the nut grass areas or dipping a paint roller/brush into Round-up and painting the weed will help prevent the herbicide from getting where you don't want it. It's also easier to do this when the nut grass is about 8-10" tall and well above the other lawn grass. Keep in mind, however, that repeated applications may be necessary.