One of the common questions/situations folks face in an ldap implementation is implementing some sort of locking mechanism for old accounts. We use the employeeType attribute in the inetOrgPerson schema. You could probably use any similar attribute (or even a custom one). One way to implement this locking is to add checks in your application code to remove such accounts. This strategy is bound to leak stuff (either due to application problems, or coding errors), sometimes this data may simply have multiple access points - through a public address book, or through folks directly accessing the directory data. A better way (imo) is to filter these accounts within the LDAP server itself. OpenLDAP allows you to set acls on specific filters. This works beautifully for cases like this. As an example, we have this acl in our slapd.conf file.
access to dn.children="ou=People,dc=mozilla" filter=(!(|(employeeType=Contractor)(employeeType=Employee)))
by group="cn=admins,ou=ldapgroups,dc=mozilla" write
by * none
This blocks out accounts with any other employeeType (other than employee or contractor) from everyone, except the administrators. Of course, this depends on you setting the employeeType attribute to some appropriate value (like retired) on inactive accounts.