One way you can do this is by using the LabelMask tool, which is in the Delineation Panel. If you press ‘Update List’ you will see all of the labels that are in the label volume. Select all of the ones you want to keep, then press Make Mask. If you then load the label file as the primary volume, you can then apply the mask to the volume (press ‘Apply’), and it will set all of the labels outside of the mask to 0. Save the primary volume as your new label volume, and then load it. It should only have the labels that you want.