If you have never paid the employee and just want to delete them, update their Employment Finish Date.
Go to Setup -> Employees, then select the employee and click Edit. Select the Employment Tab and click the Employment Finish Date. Select the required date and click Save.
If you have paid the employee, you will need to create a Final Pay to pay them any Holiday Pay or Annual Leave they are entitled to. Please refer to the following Support Centre article:
You can reinstate a finished employee by clearing their Employment Finish Date. Please refer to the following Support Centre article: