You can attach a class handler for the tree view to DropQueryEvent of the RadDragAndDropManager (with last parameter true). In the attached handler you can implement the required logic to allow or forbid the drop operation. The attached project demonstrates described solution. Also, it contains a commented code which demonstrates a slightly different approach. You can employ this one that best fits your needs.
Please let us know if you need more info or the solution is not what you were looking for.
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