trash and hydrant

I'm in California, bay area.
I have some new neighbors accros the street and they have been parking next to my house (which I don't mind) BUT they throw trash from their cars all over the side walk. I have asked them very nicely to please pickup after themselves and not to throw trash out of their cars. You can see why I'm posting here, because they are still trowing their trash all over the side walk.
We've been picking up after them but it's got to stop. I'm a corner house so my other neighbors don't see the trash unless when they drive over that way. Is there anything I can do besides telling them and picking up after them? Are there laws about leaving trash on side walks?
Also another thing is there is a fire hydrant right infront of my house and they KEEP parking their cars RIGHT THERE! No cops hang out here since it's just a little neighborhood street so they will never get tickets for parking. What can I do?? I've told them time and time agin, I have even gone as far as leaving notes on their cars and posting a sign on the hydrant. Please help me. My neighbors don't seem to give a damn but EVERYONE should give a damn. I'm not annoyed over the trash because it's on my side of the side walk but because it makes everything look so ugly and messy and this should be a nice place to live, plus many people on my block like to walk their dogs over here and I don't want dogs picking up something and eating it. Anyway, please help me with my two problems.

You say that the cops don't hang out in that area, but there is no reason you can't call them and let them know that they are constantly parking in front of the hydrant, and call when they are actually parked there and request that they come and ticket them, a few of those might stop them. Also now correct me if I am wrong but could he not set up a camra on his property to catch them littering on his property and possibly city/county property I am assuming they have the space between the road and sidewalk. Anyway, you could turn that over as well. I think that there has to be a way to stop this call you local police station and talk to the code officer they will give you some advice.

thank you very very much for that advice. I will call and find out more on what I can do. And yes, they are littering on that grassy space between the sidewalk and street.
If anyone has anymore advice please let me know! Those people are driving me mad.