@Joan2155 I think Instant Book is on by default when you set up the listing - That said I assume you would have noticed before now so it must have switched itself on. Airbnb did switch off Instant Book recently so perhaps they overdid the switching it back on again!
While you might not have turned IB on, it seems that it is on. If you are not able to accommodate the guest or do not think they would be a good fit for your listing, contact Air BNB and have them cancel. Otherwise keep the booking.