I don't know if this is exactly what you want, but if I were doing it, I'd write a daemon that periodically checks your mail server, and when it finds an email message that "matches" the criteria for a "bounced" message, takes the appropriate action on unsubscribing whomever it was you had written to.

Is that the kind of thing you're trying to do?  If so, let me know, and I'll dig up the code I wrote maybe half a year ago for doing my own "biff" (automated checking of the mail server with user notification for new messages, etc.)  Then you can modify it to your needs as far as what action needs to be taken on "bounced messages".