You can remove an employee from your account by selecting them under Setup > Staff and clicking 'Remove'.
However, we generally recommend not deleting staff because their appointment history will no longer show in your account. Instead, we recommend removing their ability to access your account and the ability for clients to book with them, as follows:
Click on their profile under Setup > Staff. Under 'Performs', select 'No services'. Under 'Allow this staff member to log into Full Slate?', select 'No'. Then save your changes. You might want to create a category called something like "Past Employees" and click and drag the employee under it. That way, inactive staff will be easy to identify.