[g]iven the potential for adverse consequences, organizations should do the following:
System access should be terminated as quickly as possible when an employee is leaving a position under less than friendly terms. If employees are to be fired, system access should be removed at the same time (or just before) the employees are notified of their dismissal.
When an employee notifies an organization of a resignation and it can be reasonably expected that it is on unfriendly terms, system access should be immediately terminated.
During the "notice of termination" period, it may be necessary to assign the individual to a restricted area and function. This may be particularly true for employees capable of changing programs or modifying the system or applications.
In some cases, physical removal from the offices may be necessary.[1]