I think I'd avoid actually disabling the account through the MTA, especially if it's for security considerations and your accounts have shell access. But it wouldn't be that hard:
My actual goal is to reduce load on the server. These are simply email users and have no shell or any other access. If there not using the account I do not want Exim wasting resources virus scanning, spam scanning(spamd) and storing messages for them.
I want to add all email users that have not checked email through pop3 or imap in ~6 months to /etc/virtual/suspended_email. Exim will reject messages to these accounts with error message: "Account suspended due to inactivity". I do not want to suspend pop3 or imap access in fact if they start using the account again I want to automatically re-enable. Likely will run a script in cron.daily to check all this.
I think that better explains what I am after.
Thanks.
Matt