Another one (may not be practical )
Electrify the corner or place where he spits, let be spit and he might be lucky enough to get a good shock
I put up a religious picture on one side of door with No Spitting in hindi written on it. Now that person has started spitting on other side of door where there is no pic.
I'm going to punch in his face him if i catch him red handed in future.
Just asking some question, based on personal experience.
1.Did you abuse anybody in your neighborhood ? or scolded a kid staying in the apartment ? or abused someone in the apartment who scratched your car/bike ?
2.Do you make lot of noise late night ?
3.Do you stay with friends or family ?
If you manage to identify him and if it turns out to be someone from your building, you can indulge yourself in a spitting contest of sought. You know, spit right back at him or at his door.
another brainstorming advice : keep a spitoon near the door and see if that helps?
or stick some plastic sheets on that side of door for a week or so then remove it