I spoke to Airbnb about this but did not reach a satisfactory conclusion.
I received a reservation (Instant Booking) from a young chap who's only review was "left the place in a complete state" so declined him and apologised saying it was due to his poor rating. I then received a text from him saying it was just for him and his girlfriend, that he'll be good and he then re-booked the same date again.
It then became apparent that unless I turn off Instant Booking (which I don't want to do) there's nothing I can do to stop him re-booking every time I decline him! If I cancel him again bang goes any Superhost ambitions. I also enquired about setting a security deposit on his reservation but that cannot be done after the booking is made. Blocking or reporting him for this type of situation was not an option either.
Does this seem like a glitch in the system to anyone? The only viable alternative in the end was to accept his booking and stress the house rules etc and hope he doesn't trash the place.