Whether the employee is seasonal or or simply no longer works for you, deleting/inactivating an employee is easy.
Go to the Employees page. From there, simply click the red "X" to the far right of an employee's name. Then select "Mark as deleted" to delete/inactivate them.
To be clear, we say "deleting/inactivating" because while this removes them from your list so you don't get charged for them, it moves them somewhere you can easily access. There you can restore the employee, complete with remembered settings and records. To access this page, scroll down on the Employees page to where you see the link that says: "Accidentally deleted an employee? Click here to restore the employee." Clicking here will take you to your "Deleted Employees," a recycling bin of sorts.
From here you can choose to either delete the employee forever, deleting all records associated with the employee, or restore them.