How old is the child? I would take very different approaches depending on the child's age.

I'd say, "HEY! What do you think you're doing? We don't disrespect other people's property like that! That's disgusting! How would you like to be walking across the yard and you get someone's lugee all over your foot? That'd be gross! Next time, swallow it or go spit in the bathroom sink where other people won't see or step on it, OK?"

If a younger child did it again, I'd assume a dental problem and check into that. If an older child disregarded me, I'd probably assign work beautifying property like pulling weeds, mowing an elderly neighbor's lawn, picking up trash in the neighborhood, etc.

I honestly wouldn't expect it to happen again after that.

always the terrible two's or three's. it was so hard raising my son at that age b/c of there lack of understanding. i was told the world is all about them at that age.
i had two girlfriends and they had terrible two's they would spit, kick, hit me, rub food on my clothes. every other minute i would say "stop"! no! stop! no!
so i stop talking to both of my friends, they was so stressed out and didnt know what to do & i had to deal with it.
im sorry i would gently pluck the kid in his lips. they need to know that it's wrong.