Offboard them as soon as you know their last day. Coverage and billing run through the end of the termination month, not their last day of work. Offboard before your monthly billing date and they won't appear on the upcoming invoice.
After that it depends on whether Thatch has already paid the carrier — if not, the amount is credited on your next invoice; if it has, the employee must request a refund from the carrier, and only a refund that actually comes back gets credited to you. If the carrier doesn't refund it, the charge stands.