Get Everything in Writing
The number of top 40 hits from annoying alt/indie/whatever band Sugar Ray would fill one side of a vinyl album; the problem is, who would want Sugar Ray on vinyl? Or at all?
Could anyone in America pick Murphy Karges or Stan Frazier out of a lineup? The "bitter" campaign to destroy their personal and professional reputations should rest squarely on whether or not anybody beyond their own families even knows who the hell they are and why they were famous 15 years ago.
Situations like this remind me of how bands start--nobody writes anything down and it all just sort of evolves and then, one day, the guy who is the most famous walks away with everything while everyone else sucks eggs and begs for someone to listen to their complaints. If you didn't get it in writing, it's gone. And whatever you do get will end up in the hands of the lawyers.
The alternative to all of this is to make music and learn from your mistakes. Did your old indie band succeed a little and make a little money? Good, now keep making music and stay relevant. Your audience might shrink but your sense of self-worth should not be tied to the asshole who rips you off. Half the time, that asshole is ripping you off because he's broke and has run out of ideas. Don't be that guy.