How to prevent former employees and administrators from accessing your server

How do we prevent our ex-employees or ex-administrator from accessing our server remotely?

It depends on the operating system of your servers as to how you want to remove access from a server or network device when an employee leaves the company. In the event that an IT administrator is no longer with the company, administrative or root passwords should be changed to prevent unauthorized access. In the case of administrative permissions for a user, they could be removed from the administrators group or their accounts could be deactivated upon their departure from the company. I would advise the company to create a policy to ensure that when an individual leaves the company that their data is backed up and their accounts are removed to prevent further access.