Set your AI to "Vicious" and then play it again.

You could mod the retreat threshold for the AI,

In your gameplay.constant file at the very bottom is:

aiData
  aiRetreatThreshold 0.25

Changing that number higher or lower will get you an AI who retreats or doesnt like you want.

Well technically if the AI retreats more it is less likely to take enough loses to force it to surrender. But from what I've noticed the AI only surrenders after you've seriously surpassed it in military strength. Even with its economy intact, I'm sure if you would have pressed your advantage you could have destroyed its economy before it could rebuild.  As mentioned, try going up a difficulty level, greater resource advantages mean the AI will constantly be producing ships, and thus shouldn't hit the surrender threshold until you have destroyed all its shipyards.

Dang it I too been wondering if there was somewhere a No AI Surrenders Mod. I like fighting my opponents to the bitter end and hate it when they surrender.  I dislike when my opponents surrender when they have an entire star system to themselves.  It makes the game more fun seeing my enemies' enemies come back from the brink once I campaign against our mutual opponent as well.  I'm a Big Game Hunter so like long games.  I hope that Stardock might consider adding a button/option in the future.