At what point do you just station an officer nearby, lying in wait, that will arrest that person on the spot? There has to be some criminal law against this. And as there is not an infinite supply of shit-smearing persons, it would stop soon.
Why assume people are just shit-smearing for "shit and giggles"? As far as I understand it, it's mostly homeless people who end up "doing their business there", due to a lack of better options.
Just fining them, when they ain't got a whole lot of alternatives, sounds like a rather mean, and inefficient, thing to do. They probably can't pay the fine anyway, but they still gonna have the very same bodily urges. So that whole exercise would only have generated some useless bureaucracy, without actually having changed anything about the problem.
A much smarter solution would be to offer homeless people an actual place to do their business, instead of trying to "police away" fundamental societal problems like poverty and homelessness.
Like if there were a public restroom near that area. It's at a subway terminal, I'm sure other people have "needs" as well. And if a homeless person needs to take a private crap, well, all the better.
The problem with "public restrooms" that have been tried in california is that they just turn into heroin dens and places for prostitutes to do their business. It's a difficult problem that has seen the occasional actual attempt at a solution
That stop has an outdoors elevator, and somebody was wiping shit all over the keypad daily. So reality is even more vile than any urban legend.