Put Captcha requirements for creating new accounts and posting content. Make the user submit to multiple tests. This will reduce the entropy of shills, spammers, and bots.
I posted a technical question to Reddit (please forgive me) and it made me go through about 5 captcha's. This was a pain in the ass, but I saw it as effective. One the other side of things, I don't particullary care for Google's services, CAPTCHA being one them, and by picking out cars, you are essential doing free labor for Google, helping assist their AI program.