The simple answer is no. They accepted the comic, they get the comic. If that's enough, you are half way done with the comic, refund them half anyways. It is not their choice, and them trying to force you to do extra work they didn't pay for is completely unacceptable. It sounds like they are just trying to steam roll you over and get everything they can out of you.

Personally I'd say refund them the remainder and call it a done deal.

If you don't want to force a refund, you can give them a choice of you finishing the comic, or they get a refund, and make it clear those are the only choices available to them. If they do not chose one or refuse, refund them.