Impossible to conquer the final city in a civ

I have my armies outside of Mohenjo-Daro. It is the last city of the Harrapans. They have no outposts.

I have conquered Mohenjo-Daro three times now. Every time I move my army to ransack on the next turn, but before it can ransack I'm forced to accept the surrender and give Mohenjo-Daro back.

Every time my war score is 75 and I need 80 to keep Mohenjo-Daro. There is no way (that I know of) to get the score up since they have no armies or outposts.

Has anyone encountered this? Have you found a way around this? I just want to eliminate the Harrappans.