It is true that you can turn off IB. HOWEVER, you can no longer vet potential guests if you do so - i.e. no profile info, no ratings, no reviews before accepting the reservation. This is apparantly something AirBnB is "trying out", so I'm told by Customer Service after discovering this myself. I was then told that if, after accepting the res and vetting their profile and having an issue or a schedule change to go through the process of cancelling. In other words, accept all and cancel later instead of reviewing and declining beforehand is AirBnB's advice. The logic is absurd and unfair to hosts and guests!
It is an absolutely unacceptable situation for the many (even if not the majority) of us hosts, especially those of us in private, single family homes who choose not to use Instant Booking so that we can vet potential guests for our private homes.