For those users I would setup and automatic email to go out maybe every 3 months to confirm user is still active and in post. If they do not reply then the account gets suspended within 30 days and archived after 60 days. Could well be just a Group Contacts setting that goes out quarterly in BCC,. Still may not be a full proof system, but may well result in a reduction of redundant accounts. Best of luck with whichever way you plan to attack the problem. :)
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