Thank you for writing.
You can subscribe to the CollectionChanged event of the RadAutoCompleteBoxElement Items collection. In this collection are stored all the tokens that have been built. Here is the code:
.radAutoCompleteBox1.TextBoxElement).Items.CollectionChanged += Items_CollectionChanged;
sender, NotifyCollectionChangedEventArgs e)
(e.Action == NotifyCollectionChangedAction.Remove)
//Handle block remove
I hope this will be useful. Should you have further questions, I would be glad to help.
the Telerik team