public class MyNTreeViewEx : NTreeViewEx
{
    protected override void ApplyDragDrop(NLightUIItemDragDropEventArgs e)
    {
        NTreeNode over = e.DragOverItem as NTreeNode;
        // condition which determines that this is the node(s) that cannot
        // drop another node on it.
        // In this case you can set Tag property of the node to true,
        // or you can choose another approach.
        if ((bool)over.Tag == true)
        {
            return;
        }

        base.ApplyDragDrop(e);
    }
}

myNTreeViewEx1.ItemDrag += myNTreeViewEx1_ItemDrag;
...
void myNTreeViewEx1_ItemDrag(object sender, NLightUIItemDragDropEventArgs e)
{
    NTreeViewEx treeView = sender as NTreeViewEx;

    // get the hovered node
    NTreeNode node = treeView.VisibleNodeFromY(e.MousePosition.Y);
    if (node == null)
    {
        return;
    }

    Cursor cur = e.Cursor;

    if ((bool)node.Tag == true && e.Cursor == null)
    {
        e.Cursor = Cursors.No;    
    }
    else
    {
        e.Cursor = cur;
    } 
}