You may be one of the new hosts who are part of a trial that means you can't switch off Instant Booking.
Obviously it leaves you open to risk, and undermines the whole concept of developing a conversation with someone coming to stay in your home. However, it drives better revenue for AirBnB as it leads to impulse buys, etc..
Some hosts have indicated on other threads that the only way to get around this is to keep opening and then closing your account until one pops up with Instant Booking as a choice, rather than mandatory. Not sure where this stat has come from but the trial is with around 25% of new hosts.