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" the employee because this removes them from your employee list, meaning you won't get charged for them in the future. It also moves them to a recycling bin of sorts that you can easily access. You can restore inactivated employees, complete with remembered settings and records. To access this page, click Employees > "View Full Employee List" > then scroll down on the Employees page to where you see the link that says: "Restore Employees." Clicking here will take you to your "Deleted Employees" page.
From here you can choose to either delete the employee forever, deleting all records associated with the employee, or restore them.