Well, I see that TabContainer doesn't have such feature, it raises event after the active tab is actually changed, and unfortunately there isn't event like activeTabInedxChanged that assumed to fire before changing with ability to cancel action.
But, fortunately Ajax Control Toollkit js-control behaviors is quite well organized and designed (I really like they idea with js-class hierarchy like the .net controls), so that you have opportunity to inherit from AjaxControlToolkit.TabContainer js-class and extend it with such an event.