I have to agree with Kendra and Heralde, while it would be wise if people did their best to NOT do certain things like pollute the atmosphere (and even littering for that matter), but actually being told not (and making it a sin in this case) isn't going to do any good because it's true: people DON'T like being told what not to do.
I remember in Grade 3, our teacher actually made a point out of that people, psychologically always want to do the exact opposite of what they're told (reverse psychology) when one day, being sick and tired of the entire class whispering, or talking quietly amongst themselves when we're supposed to be quiet, he finally said "Alright, that's it, go ahead! Talk! Talk all you want!", and suddenly the entire class was dead silent, hesistant to say anything, thus proving his point that people always want to do the exact opposite of what they're told to.