Deanna: First of all: yes InstantBook is terrible, no argument there. However AirBnB is determined to keep pushing it, because... fighting racism. (Not a joke, actually)
As for setting a minimum stay period while discussing a potential long booking: you can set minimum stays only for that period only. So your regular minimum stay can remain at 1 day, but when discussing an inquiry that's much longer, say from 15 November to 15 December, then you can define a separate minimum stay of 30 days or something for that period. That effectively blocks it unless you'd get someone else for that long a stay during that time.
And hten I would contact AirBnB to get the option to turn InstantBook off for your listing. AirBnB esperimented with removing the option for some hosts (without telling anyone, not even the people in their own help desk initially) but I think most people at AirBnB are by now aware that they either treat all hosts equal or they end up losing a lot of hosts. So just push them for it. Or if all else fails: close your account and sign up again.