You need to call AirBnB Customer Service. As @Brian_R170 said, another person on this forum reported that while she did NOT enable Instant Book, there is apparently a glitch in AirBnB’s software that allows some guests to instant book anyway. When you call Customer Service explain that you know that this has happened to other hosts, and that you don’t want to use Instant Booking. Install the app on your smartphone and see if your listing shows Instant Book available since apparently those instant bookings were coming from folks booking on smartphones. Also read the post by the other user that had this problem, just click here → Instant book free cancellation - #5 by cjmart
However, why do you want to turn away business? Most of the hosts on this forum use Instant Booking because it places your listing higher on the page when guests are searching for rooms. You can always review folks after they book, and yes, you get 3 no penalty cancellations, but you can also get Customer Service to cancel for reasons like too many people, won’t obey your House Rules, etc.
Please browse through the posts on this forum to pick up tips on how to use Instant Booking effectively.