Currently this behavior is hardcoded in the RadListBox control, but we will do our best to change it for the upcoming SP. You could set the RadListBox.CanKeyboardNavigationSelectItems property to False. In this way the keyboard navigation will change move the focus, but not the selection. You could track the pits item here - http://www.telerik.com/support/pits.aspx#/public/wpf/11904
Hope this helps.
the Telerik team