@Wj3 In the olden days I would've suggested getting Airbnb on the phone, but by most accounts they aren't doing much customer support these days. So to take matters into your own hands, I think the first thing you should do - immediately - is change the door codes.
Then, you'd need to inform the guest that the door codes have been changed for security reasons, and if he still wants to keep the reservation, he will have to schedule a time to complete the check-in procedure in person. In the unlikely event that he does so, you have many reasonable questions to ask about his intentions with the property (which are quite murky at best) and a chance to negotiate yourself a way out of this booking.
If you don't reach a resolution easily, you can send the guest a Change Booking request, advancing the checkout date to today and refunding the remainder of his stay. It's hard to fathom why the guest would refuse an offer that puts money directly into his pocket. But if he does, your last resort is to get Airbnb on the phone (grab a book for the long hold time) and show from the correspondence that this was a de facto guest cancellation. I hope it doesn't come to that, since results are wildly unpredictable these days.
