Since I don't have a t-shirt that says "GO AWAY and take your clipboard with you", I've started saying "I'm not signing petitions these days, but good luck anyway."
So far, no negative reactions. If I do get any — well, either I'll just walk away or it'll be interesting to see just how in-your-face hostile this low-key, non-confrontational, introvert can get when the topic is "leave me the hell ALONE!"
Yes, I think California's proposition system is broken — unwieldy, inefficient, and subject to being gamed. No, I don't know what would work better.