what are the legal rights of a person seaching for a job and their former employer has told the other businesses that he would not hire them?

As a general matter, an employer has a right to express its opinion about its current or former employees to a prospective employer who calls for a reference check. Opinion is a form of speech protected by the First Amendment. If the employer makes a factual statement that is false and harmful, it may be actionable -- but not a statement of opinion.