In order to disconnect the synchronization with the CurrentItem I would suggest inheriting the RadListBox control and adding the following lines of code:
CustomListBox : RadListBox, ISelectorInteraction
This will do the trick. Meanwhile, I added a PITS item as a feature request for adding a new property in the RadListBox control. You could track its progress and vote for it here: http://www.telerik.com/support/pits.aspx#/public/wpf/13764
Hope this helps.
the Telerik team