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I am sorry to learn of this situation.
You can try suing your landlord in small claims court for breach of your implied "right of quiet enjoyment" - you can do this without a lawyer (the California Courts have a very good self help site: http://www.courts.ca.gov/selfhelp-smallclaims.htm). You will want to carefully compile all evidence regarding the matter - what evidence of harassment you have, and be prepared for him to show defenses (for example, if you did in fact have refuse outside your unit door in violation of your lease). I do not know what the outcome of your lawsuit will be - court cases are "inherently risky" as there is always a chance you can lose even a very strong matter - but it certainly gives you a chance to do something about this matter without having to sit and wait for someone else.
If you are afraid of imminent physical harm to yourself, you can also file for a civil restraining order (but this requires you to show objective (meaning someone else would see that there is actual threat of injury) evidence to you) - if this is the case - follow this link: http://www.courts.ca.gov/1044.htm
If you are in fear of immediate harm - please do not hesitate to contact the police again. Use 9-1-1 for immediate threats to yourself or others.
Try the small claims action - keep records of his behavior and document this as well as possible.
If you can keep video records or photographs of behavior to use in support of your claim, you can do so.
You are welcome (although I would recommend taking a second look at small claims - the filing fee is very small, and you can even apply for a filing fee waiver).
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Thank you again, and again I wish you the best.
If you have already exhausted your other options to date (attempted mediation, attempted legal aid, attempted negotiations, attempted police reports), you can try small claims. But those are the remedies that exist to deal with these situations.
You can consider paying a lawyer for an hour of their time to review your case with you and perhaps come up with a strategy to deal with it (remember we can only provide "general information" on this site - a local attorney could review all of your documentation and help you come up with a specific course of action. But at some point you would probably end up having to sue this person anyway. You can find local attorneys using the State and local Bar Association directories, or private directories such as www.AVVO.com; www.FindLaw.com; or www.Martindale.com (I personally find www.AVVO.com to be the most user friendly).