Daniel, there is a place in your listing settings where you can opt to not block the calendar when a guest cancels. You can also go back into your calendar, highlight the days you want to be available and click that in the box that shows the dates. Play around with your calendar page and you'll find it.
My other suggestion is that you seldom if ever, agree to cancel a reservation! If you are going for Superhost status, it negates your possibility of acceptance. Instead, agree with the guest that they can cancel, but ask them to do it. Otherwise it affects your rankings.