You have three options.
One, you either try and satisfy your buyer by completing the work, although it sounds like your buyer is not accepting of this.
Two, you cancel the order. This will automatically refund the buyer, but your completion stat will take a hit - and your buyer walks away with free work.
Three, you fight the cancellation through customer support. You will need to demonstrate that the work you delivered is the same as what your gig promised.
Some other quick thoughts… never offer unlimited revisions. In extreme cases, abusive buyers will come back to you time and time again for no money. Also, most buyers who cancel an order after delivery simply want free work. No doubt some buyers are genuine and cancel for other reasons.