There are numerous ways in which an employee could breach their employment contract, including failure to do their job as required, gross misconduct or breach of express terms such as a restrictive covenant preventing them from working for someone else.
The employer has several options, including dismissing the employee if the breach is fundamental, suing the employee if the employer has sustained a financial loss and asking the court to grant an injunction preventing the employee from working for a competitor in breach of the contract terms.