I have instant book on at present although I host in my own home. I love hosting and I've so far had very good experiences. But like others on this thread I'm daunted by the lack of resources for blocking people you don't want to stay. Some people are perfectly good guests, there's no reason not to recommend them to others, but you just didn't enjoy hosting them.
Also, I noticed that some guests have received distinctly negative reviews but are still able to use instant book despite having the 'recommendations from other hosts' feature turned on. I expected that this feature would mean that guests could only Instant Book if they had been recommended by all the hosts they stayed with.
The only choice is to remove Instant Book, and that would be a shame as it makes life easier for hosts, guests, and Airbnb.
So please hear us, Airbnb!