The remedies for not following the employment contract should clearly be listed in the contract. For an independent contractor or consultant, in addition to the remedies in the contract, the general remedy would be to file a civil suit in a court of law under the theory of a breach of contract.
Additionally, for an employee who is employed under a union contract, the employee can file a grievance with the union. If there is no union involved, then depending on the type of violation, the worker may be able to file a complaint with a government agency. A local employment attorney can better advise you on the options available to your for your specific case.