In the biggest rebellion yet against a health insurance company, more than 400 doctors in Dallas have terminated their contracts with Aetna Inc.'s health maintenance organization, and Aetna has retaliated by cutting off their access to all its Dallas patients. As a result, nearly 30,000 patients covered by Aetna must accept doctors who remain under contract to the company. If the patients choose to remain with the insurgent doctors, they face paying them cash for their care, at least for the three or four months it will take the patients to sign up with other companies.