I've found that the only way to practically guarantee a AI surrendering to you, is to have an overwhelming coordinated offense take several worlds in rapid succession before they can bring their forces to bear.
I get the impression that the AI checks to see if there are any other AIs that can stand up to such an array of forces to ensure the preservation of their kind, and if none are available, they're stunned into complicity. If all you've done is prostrate their fleet without actually taking any worlds in the process, they'll surrender to someone else just to give you the finger.
I only found this out because the damn UI froze on me for a moment after I conquered the Iconians' ninth consecutive world and in the Planetary view the "Done" button hovers right over the "Turn" button at 1024x768. The turn immediately after, all their remaining worlds flipped to me, and Iso's reaction was one of pure shock.