At this point, you do nothing. Your guest must process this own cancellation. Absolutely DO NOT cancel on your guest's behalf - you will be subject to steep penalties.
Once you guest cancels the remainder of their reservation, Airbnb will process any and all amounts they may be entitled to per your cancellation policy. Since you have already receive the full pay-out, Airbnb will deduct the refund portion from your future payout(s).
You don't need to worry about issuing a refund yourself - Airbnb will take care of all the finances.