@Sally155: Any time you cancel a guest your calendar is automatically blocked. If you receive an instant booking and want to cancel it you need to call Airbnb and have them do it to prevent the blocking of your calendar, monetary fines, loss of position in the search ranking and loss of SuperHost status. (Airbnb takes cancellations VERY seriously.)
In your case, since you've already hit the cancellation button yourself you can try contacting Airbnb and see if they will unblock the calendar since it was an instant book. However, you will still need a reason that you cancelled the reservation. Simply because they instant booked probably will not float.
One more thing, you need to go into your listing and confirm that instant book did not accidentally get turned on. Go to your admin panel, click "Host" then "Manage Listings." If there is a little lightning bolt to the right of your listing next to the "Preview" link then instant book is currently on. Click the bolt and turn it off on the next screen.