When you connect your Airbnb listing to a PMS like Host Tools, Airbnb enables instant book on all connected listings.
However, you can easily disable Instant Book by following these steps.
Go to Airbnb.com, or open the Airbnb app.
From the main menu choose "listings".
Choose the listing you want to disable Instant Book on.
Go to "Booking Settings."
Go to "How guests can book," and toggle "Instant Book" to off.