The events of the DragDropManager are not public and they cannot be used with the EventToCommandBehavior in XAML. What I've meant in the previous post was that you will need to pass an instance of the UI element to the ViewModel in order to be able to handle the methods there and this would break the MVVM pattern. Other solution would be to use an attached behavior which hooks the DragDropManager methods to the desired UI element. Inside of the handlers you can get the DataContext of the UI element and execute the needed logic in order to prevent or not the drop operation. For your convenience I have prepared a sample project which demonstrates the exact approach using ListBoxes, please check it and let me know if it helps.
I hope this work for your scenario.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.