In my city, its actually illegal to spit (yes because of TB... which isn't just a "back in the day" illness, its still very much out there) in public. Of course its not really an enforced law - cops don't go around ticketing spitters. But the point is, perhaps look into if spitting is illegal (or counts as littering) in your area, and if so, send around a note to your neighbors about spitting in the area. I would keep it general, perhaps "there have been complaints", so as to not single the guy out. If you rent, perhaps contacting your landlord with the complaint would be easier.
But if its not illegal... well unfortunately it might be something you just have to deal with. I doubt he's doing it for pleasure - its probably more annoying and frustrating and gross to him then it is to you.