If the label generator returns null for the label, then the pie chart doesn't display a label for that section. So you can achieve your result like this:
StandardPieSectionLabelGenerator labelGen = new StandardPieSectionLabelGenerator(
"{0} ({2})", new DecimalFormat("0"), new DecimalFormat("0%")) {
@Override
public String generateSectionLabel(PieDataset dataset, Comparable key) {
if (key.equals("dormant")) {
return null;
}
return super.generateSectionLabel(dataset, key);
}
};