Three ways to go. All are bad.
1. Super massive and super annoying Captcha implementations. Everyone gets to prove that they are human every thirty seconds. A Captcha Co. Representative will arrive at your home for a thorough cavity search to ensure that you are indeed human - the Representative will be singing The Killers' Human during this process. He then installs a peripheral device which will be used to collect blood samples and take retina scans whenever your computer does something slightly inhuman. While this has the added benefit of making access to furry pr0n very inconvenient, the downside is that it's just ten more hoops to jump through when signing up for or accessing the site.
2. IP blocking. Why use Occam's Razor when he's got a really big cleaver? Money's on a bunch of these bots coming from the same places. Rather than blocking the individual IPs, blocking entire chunks of countries can be far more effective. The only downside to this one is that it denies access to legitimate users.
3. Step up staff presence. Enlisting a zealous brown shirt Hitler youth anti-bot force that will race to delete posts from bots and ban their accounts will make sure higher staff can do something more productive with their time, like making sure the new recruits aren't trolling others and abusing their new powers. Can't possibly imagine a downside to that one... >.>
Perhaps stepped down implementations of all three would be a good approach.
