Yes, they can do this. There is no right to have your contract renewed after it expires: except *while* a written employment contract is in effect, all employment is employment at will--an employee may be terminated, or his/her employment otherwise not continued, for any reason, including telling your employer that you will not work to the best of your ability. Therefore, when this contract is up and you are no longer protected by it, they can refuse to renew it or your employment.