ASCAP

Under copyright law—I think it’s Section 155—an author actually has the right to tell ANYONE, even Sony Music, they can't represent them. Especially if you've already registered your work somewhere else, like through Creative Commons. Before a gig kicks off, you just sit down and draft an agreement (whether you're acting as the organizer, the creator, or whatever) where you explicitly state that you're refusing representation to anyone else, and just like that, you're set.
It's honestly pretty straightforward!