First off, I know squat about the stock market, so take that into account, but here's my thought.
I'm going to assume that insider trading is actually unethical and a violation of contract between the traders and their clients. So based on that, if I suspected that there was insider trading going on, I would contact my DRO and have them investigate, and if they found guilt, they would then proceed with a legal case against them.
I guess my question may more accurately be "who determines the rules of the stock market without a government regulatory body?"