If there is no notice provision in an employment instrument that dictates how much notice a resigning employee should give, they will need to give you a reasonable period of notice.
If the notice requirement arises under a modern award, enterprise agreement or legislation, you could prosecute the employee for breaching a statutory requirement.
If your employees work under a modern award and they fail to give the required notice, all modern awards require them to forfeit the amount they would have been paid if they had served the proper notice period.
Generally, a fixed term employment contract contains a legally enforceable promise by an employee to serve you as an employee for the term of the employment contract.
An employee who has resigned from their employment, but was forced to do so because of your conduct, can treat the termination of employment as if the employer dismissed them. This is called a forced resignation.