You don't need a letter, you need to file for a Restraining Order. I've provided a link from the California Courts page. It will answer many of your questions. Keep your documentation, you'll need it.
A cease and desist letter is about as effective as using a garden hose to put out a fire in a commercial building.
Follow the suggestion about a civil restraining order
The above is not intended as legal advice. The response does not constitute the creation of an attorney client relationship as this forum does not provide for a confidential communication.
The letter can be written by any attorney familiar with California's civil harassment statutes. Sometimes such a letter is effective and sometimes it is not. Assuming that a letter is written and the harassment continues, California has specific laws that permit the issuance of a restraining order. The circumstances would need to be reviewed with an attorney to determine if the conduct falls within the statutory basis for issuance of a restraining order.
The above is general information only. It should not be considered as legal advice applicable to your specific circumstance. Reliable legal advice applicable to your situation cannot be given without a consultation. The above response does not create an attorney client relationship nor should it be considered as a privileged attorney client communication.