I really don't like the idea of requiring a login to unsubscribe. Someone not familiar with mailing lists who was manually added by the owner to a premium group could get confused.
Here's an alternative that might solve the problem. When someone clicks Unsubscribe, they go to a page that asks:
Are You whoever @ wherever.com ?
Button 1: YES, Unsubscribe me from Groupname!
Button 2: NO, I am someone else.
If they click NO, they go to a login screen, and for bonus points, if that user is in the group, ask again if that user wants to unsubscribe.
That won't stop someone malicious, but malicious unsubscribes probably don't happen as often as accidental ones.