Legal Question in Real Estate Law in California

Such a suit might have merit. Possible legal theories could include "intentional interference with contract (and/or prospective economic advantage)" I don't think "slander of title" fits. However, there are some practical issues that should be discussed with a lawyer before jumping into a legal battle with this nut. These include whether any of his complaints had any justification, whether the neighbor might do something rash, the quality of your proof of facts (do you have everything in writing, including police reports, etc.?) and the ability of the neighbor to pay any damages you are awarded. There's no point is suing a guy who is broke.

It sounds like he has money because he owns a house in the neighborhood. My hope would be that you kept in contact with the tenants, and any prospective purchasers, before you sue, who would be able to testify as to this neighbor's conduct.