So, thank goodness this didn't happen to me, but I am curious how other hosts would handle this. A story related to me involved a guest who booked for a week in a house and then, four days into the stay, suddenly cancelled. The cancellation message came in around 2pm when the owner of the home was at work. She apparently sent the usual "sorry it didn't work out" message and wished them the best. Not sure if any specific language was exchanged about them leaving immediately, but it sounds like it was implied that they would do so.
So, the host arrived home from work about 6PM and found the guests still in the home, not packed at all, apparently having dinner acting like they were still registered guests. The host of course asked what they were doing and they replied that oh, they would leaving around 8 or 9 the following morning. I am not sure how she handled it after that, but apparently they stayed in the house for a free night and left around 10AM, even later than they said.
My understanding of people who cancel during their stay is that they must leave immediately. I had only one case of this, where a couple of mine, a few months ago, cancelled at around 3PM when they weren't even in the house, then came back around 6 to collect their things. Because of that experience, and this story from this other host, I am thinking of adding a notice to my check-in message that if someone cancels during their stay the door code will be immediately turned off and they will be required to leave the house without delay, maybe even suggesting that they should not cancel until they are already out the door so they don't get locked out.
How would other hosts handle this?