California Code, Health and Safety Code - HSC § 1379

(a) Every contract between a plan and a provider of health care services shall be in writing, and shall set forth that in the event the plan fails to pay for health care services as set forth in the subscriber contract, the subscriber or enrollee shall not be liable to the provider for any sums owed by the plan.

(b) In the event that the contract has not been reduced to writing as required by this chapter or that the contract fails to contain the required prohibition, the contracting provider shall not collect or attempt to collect from the subscriber or enrollee sums owed by the plan.

(c) No contracting provider, or agent, trustee or assignee thereof, may maintain any action at law against a subscriber or enrollee to collect sums owed by the plan.