there should be some criteria for the endgame that will allow the AIs to surrender after the game is clearly in the player's hands
My favorite solution for that problem is autopilot. If you don't know, autopilot lets you click a button and have the AI enter orders for you.
So the way you can use it is this: When you get to a point where you've overwhelmingly won, start clicking autopilot. If you have, then autopilot will sail you to victory in no time.
If you start using autopilot too early though, you'll see things start going bad. At that point you can stop clicking the autopilot button and try to turn the bad situation back around to the point where you're willing enough again that you can re-engage autopilot.
I realize it's a member feature but if you haven't, give the free trial a spin and let me know if you feel like this is a good solution to the problem you've brought up.