If they hadn't gone back on their word, I would advise to continue working on the comic. But at this point it sounds like they're trying to bully you into doing more work than they paid for, which is not fair at all.

Refund them ASAP and walk away. If they decide to try and send the money back, do as vauvakolibri said and advise them that if they return the money, you'll be considering it a donation. Screencap everything, and if they continue to throw a fit, post a beware. Tbh, I think it warrants a beware already.