Some landlords have a practice of retaining security deposits without explanation and in blatant violation of the law. There's only one way to deter them in this practice and that's by challenging them in a court of law.
TENANTS TOGETHER, a state-wide tenant's organization, has a downloadable, step-by-step guide covering procedures to take to get your security deposit returned including filing a small claims court proceeding if the deposit is not returned.
See: http://www.tenantstogether.org/article.php?id=3409
However, even if you win a judgment against the landlord, s/he may not pay the court-ordered judgment. Your option, then, is another legal proceeding and involves placing a judgment lien against the landlord's real property. A judgment lien is valid for ten years and can be renewed until the judgment is paid.
For information about judgment liens, see: http://www.nolo.com/legal-encyclopedia/judgment-liens-california-46807.html
