I'm very sorry to hear of your dilemma.
Unfortunately, even under CA employment law---which is perhaps the most pro-employee law in the nation, certain actions are not protected against---even in the workplace. The law will not micromanage manners and attitudes except in rare circumstances. And what your HR office meant when they blew you off by claiming that the definition of "hostile work environment" wasn't met by the offending co-worker, was that you are not being attached, harassed or discriminated against based on your race, color, national origin, religion, sex, age or disability.
Many people are under the mistaken impression that if they are not treated well at work, or if they are gossiped about or berated by fellow employees or their boss, that as a consequence they are being subjected to a Hostile Work Environment and have a viable legal claim as a result. Actually, that is not the case.
The US Supreme Court has held that harassment and hostile environment laws were not meant to create a code of civility within the workplace. The phrase “Hostile work environment" is legal terminology, and relates to discrimination which is federally prohibited—race, color, national origin, religion, sex, age or disability. Absent proof that the hostility you complain of relates to one of the prohibited acts of discrimination, then I’m afraid that you have little solid ground to stand on with regard to legal redress.
You have not articulated what it is that you are afraid of from this person, and you seem to equate his trash talk to the likelihood that it will escalate into something happening to you. While this is often a distinct possibility in intimate relationships and marriages---and is a part of domestic violence---in the workplace it tends not to escalate to physical violence.
You have done what you can do---in terms of reporting the behavior, and the company is now on notice and can be vigilant about interactions between the two of you. However, unless the company is willing to take action based on company policy which should prohibit such behavior, there is no legal remedy at present which you may use to stop the attacks---I'm sorry.
I wish you the best in 2012.
I understand that you may be disappointed by the Answer you received, as it was not particularly favorable to your situation, nor was it what I sensed you were hoping to hear. Had I been able to provide an Answer which might have given you a successful legal outcome, it would have been my pleasure to do so.
Because I help people here, like you, for a living---this is not a hobby for me, and I sincerely XXXXX XXXXX abiding by the honor system as regards XXXXX XXXXX I wish you and your family the best in your respective futures.
Would you be so kind as to Accept my Answer so that I may be compensated for assisting you? Bonuses for greatly informative and helpful answers are very much appreciated. Thanks Again,