What you describe might be a form of defamation. Defamation is also known by its common names of slander or libel. Defamation is proved if you can demonstrate that someone made objectively untrue communications of past or present fact that the person knew was untrue or recklessly disregarded the truth of the communication, and such communications caused you to suffer reputational damages.
If you think that such has occurred, your options are directly calling the person to cause them to stop, hire an attorney to write a cease and desist letter threatening a lawsuit, or just bringing the lawsuit. I would suggest starting least with the least aggressive.
Good luck to you.
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As Attorney Pederson suggests, an aggressive approach may not be the best one. I have seen judges sanction plaintiffs for bringing a case that appears to be about mere "gossip." You may wish to speak to an attorney if you have facts to support something more than gossip.
Be wary, however. If you file a defamation lawsuit, under California's "anti-SLAPP" laws, you may end up paying the defendant's attorney fees. Be sure your attorney is familiar with the SLAPP defense, which would be the expected approach if the defendant was able to claim the statements -- even if apparently defamatory -- were mere opinion, and not actionable defamation.
On the bright side, where a wrongful interference with accounts and/or business opportunities can be proved, there may be a viable legal claim. An attorney will need to determine whether or not the interference is wrongful or not based on all facts and circumstances.
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