Just wait until the alliance treaty runs out. It works for 99 turns, if I'm not mistaken, and ends automatically once the time is up. You could also use Curgen's Volcano to end the game. Find the enemy sovereign and cast CV on him/her. This will automatically kill the sovereign (no respawn) and end the game for you. Well, you could probably consider this an exploit, because CV isn't supposed to do that, but the choice is there.
