Unfortunately currently there is no way to cancel a user selection. In the Q3 release of the controls the TabControl has the PreviewSelectionChnaged event that can be handled to cancel the user selection.
When you cancel the selection, you can see what selection was intended and store it somewhere. Then you may decide to navigate again to this tab after the user closes the dialog.
Unfortunately currently you have to handle the SelectionChanged event and switch back to the previous selection, which may not always be desired.
the Telerik team