I was thinking this morning about asking one of my friends to translate this into Korean, and making a few hundred cards:

“Spitting on the street spreads colds, flu and diseases like tuberculosis and hepatitis. Korea has one of the highest rates of tuberculosis infection in the world. Stepping in spittle on the street brings germs and viruses into the home. Every day, Korean children and elderly people die, because you and others like you feel an irresistable need to spit on every horizontal surface you see. If Korea is ever to enter a community of modern, civilized nations as a true equal, behaviour like this has got to stop. Please, think of the children!

and handing a card to every guy I see horking an oyster on the pavement or platform or stairs or any other public thoroughfare. The thing is, I suspect that would push me over the edge from amused, sardonic observer into raving crank. I also suspect that I’d go through a few hundred cards over the course of any given weekend.
I do like the way I’ve managed to work in a Metafilter injoke though. Almost makes it worthwhile.