If an employee leaves your organization, we either can rename the account for the employee who is going to replace the employee who left or delete the account. If the employee is being replaced immediately it is better to rename the account. If that employee role is removed permanently, it is better to remove that account for security purposes. We can disable the user account also, if in near future someone is going to replace the left employee.
1) To delete an Active directory domain user account, open the Active Directory Users and Computers MMC snap-in, right click the user object and select “Delete” from the context menu. Click “Yes” is the dialog box “Are you sure you want to delete this object?” to confirm the deletion.
(To open Active Directory Users and Computers MMC snap-in (DSA.MSC), select Start > Administrative Tools > Active Directory Users and Computers or type DSA.MSC in the run dialog box and hit enter)