@Nino15, the guest was able to book because you have Instant Booking turned on. If you are a new host this is likely to be because Airbnb are running a trial where Instant Booking is turned on by default and you can't turn it off. It's a terrible idea and goes against the whole ethos of hosts being able to ingterract with guests before they book to make sure there is a good fit.
But don't cancel the booking unless you really really have to. Airbnb hates guests cancelling and imposes penalties. The dates will be blocked out so other guests cannot book and you might incur a financial penalty as well. If the guest called you and asked for is money back that indicates that HE wants to cancel, so let him do so through Airbnb and he will get a refund based on your cancellation policy.