The other day I noticed a polite sign in Yokosuka. The sign uses the word "refrain". It means that when a person wants to do something, he should stop and not do it. I don`t think that this word is good for this situation because it is a little too polite. For things that are against the law, stronger words should be used such as PROHIBITED or NOT ALLOWED.
I think the bottom part is funny because it says ILLEGAL littering. Is there LEGAL littering? No, of course not! Additionally, it says "serious offenders" which sounds like it is ok to litter a little bit.
By the way, what do you call a person who litters? A LITTERBUG!
In my country, you can see many signs like this along the road.