You could easily achieve the desired by implementing a custom ListBoxDragDropBehavior which inherits from the default ListBoxDragDropBehavior. You will need to override the CanDrop method in order to check whether the drop will be inside of another control and if so to return false:
For more details about the ListBoxDragDropBehavior please check the following article from our online help documentation:
I have also attached a sample project which demonstrates the exact approach.
Hope this will work for you.
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