@Bea77 Yes, tell the guest that they can book one more night if they'd like to stay past check-out time. It doesn't matter that they leave before midnight, as their "extension" still prevents you from accepting another booking for that date.
If they have the audacity to suggest that they shouldn't be charged another night because it's the same calendar date, kindly show them the exit and make sure they've fully departed by noon.
some people obviously don't understand that in this business the days / nights doesn't count from midnight to midnight but from check-in time to check-out time.