A vital process and one of the most vulnerable times for a business. All efforts should be close down the the service as effectively as possible. In addition communication should provide for termination responsibilities of the employee as set out in the terms and conditions of employment. There is very little that is stated explicitly in the standard but here are a few things to consider:
Return of company property (phone, laptop, door passes)
IT to remove access to network both remote and hard connection protocols.
All passwords to be removed.
Consider monitoring the email account for stray emails from customers/suppliers.
Responsibilities assigned especially any considering information security. Affected customers or suppliers informed as to the change of status.
Where the leaver has operated in the IT or similar department, immediate action should be taken to ensure access to firewalls, switches, routers, network (internal and extranet) spaces, cloud access, processing software and the company website and any social internet presence. This list is not exhaustive!