Google's Andrey Lipattsev, a Search Quality Senior Strategist, said yesterday at the 42:40 mark in the Google Q&A via WebPromo video, based on a question from Rand Fishkin, that Google is exploring using machine learning with web spam. Andrey said, "I can say we have definitely been looking to use machine learning across the board, including web spam, including other areas."
He then went into more detail saying he "cannot got into any particular huge success we had," with machine learning but he is "quite confident that we will continue exploring." They are not using it today in Google but Andrey added, "I am quite confident that the effects of trying to apply it has been very beneficial."
So I would expect, especially in a RankBrain world, Google will be making spam detection and search quality more machine learning based in the near future.