private bool RecursiveMethod(TreeNode treeNode)
{
   if (foo == bar)
   {
     MessageBox.Show(treeNode.Text); // I don't want to continue my recursive search!!
      return true;
   }

   foreach (TreeNode tn in treeNode.Nodes)
   {
      if (RecursiveMethod(tn)) return true;
   }
   return false;
}
( and update the calling method to match)