No expansion without equilibrium. No conquest without control. Pursue success in serenity. And service to the tau'va. With pride. -- Shas'o T'au Shi'ur
Once you've destroyed all of an AI's production structures they'll sell all of their remaining structures and attack you with the rest of their units.
However if you are playing with the 'Short Game' setting, once the AI sell all their structures they'll automatically terminate themselves thus an equal to a human player's option to surrendering.