If an employee opts-out within their opt-out window, your payroll contact will be notified overnight by email.
Below is an example of the email you'll receive.
If the employee opts out after their opt-out window, your payroll administrator contact will be notified at the point they finish the next upload. Contributions will be expected for the existing period, with the cessation taking effect from the beginning of the next payroll period.
The Payroll Changes Report can be found in the Employer Payroll List under the Home tab.