I have a suggestion, could the spam filter separate the messages which
people mistakenly try to unsubscribe with and a script run that emails
them back saying:
"This is an automatic email sent out by the unsubscribe function.
Your email to unsubscribe has been unsucessful due to the fact that it
was sent to the wrong address.
Please email firstname.lastname@example.org to complete the
Gentoo lists management"
Or something similar.
email@example.com mailing list