Allow cancelling a drag operation originating from certain items - basically the customers need to disable the drag operation on some items.
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Here is a possible solution with handling DragInitialize
 
DragDropManager.AddDragInitializeHandler(this.tree, this.OnDragInitialize, true);

 private void OnDragInit(object sender, DragInitializeEventArgs e)
        {
            if (((e.OriginalSource as RadTreeViewItem).Header == "B1")
            {
                e.Data = null;
                e.DragVisual = null;
            }