How could we stop people throwing garbage?

So many students in SPPA dirtified the library by throwing away the garbages from what they ate. To be honest, I was one of them... but just twice:) Anyway, teachers are concerned about this problem and maybe I could give them some suggestions.

I think maybe teachers could ask students to pick up the litters before the class end. I know this isn't a happy thing for students but maybe it could help....

Or, I think the best way is to give detentions to students if they are caught littering because I hate detention.

But, I know it could be hard to catch someone littering. We can also make posters persuading students to not to litter around. 

Posters like these could stop littering :)

Or posters like "Big Brother is Watching You So Stop Littering" might help

I also think littering needs to stop because it does not only affect the school, but it also affects the world because the habbits of throwing litters on the ground will never stop even outside of the school. So, I will also change the habbit..:)