Well, you have different solutions with the RadListBox to obtain your desired result.
For example, you can use ViewState to create a copy of your RadListBox before any user input. Then, on SelectedIndexChanged method, you compare which value has changed to get the ID of the item that was just unchecked.
Other example, you can use client-side event (in addition to Eyup link) with Ajax Request to execute server-side method, like SelectedIndexChanged. More information here : http://docs.telerik.com/devtools/aspnet-ajax/controls/ajax/client-side-programming/overview.
Let me know if you have any questions.