There isn't an option for that on DockManager however once a document window is activated, you can walk up its parent control chain to look for a TabStrip. Once you find it, set its AutoInternalDragDrop property to false.
AutoInternalDragDrop is already false. Besides that setting it to false causes TabStripPageTabDragStart to be fired. But there is no means to cancel drag&drop from this event. And I don't see any other drag&drop event being fired.