The problem with this idea is how easily it could be abused against legitimate posters. One could probably say that past around ~50 posts we disable said additional functionality, but that's just more overhead.
Stopping them from getting in the door is always more efficient, and -- so far at least -- this "CleanTalk" system has done wonders where even recaptcha was falling flat on its face. I'm not 100% sure we're not blocking legitimate attempts at joins, but again that's where my contact form as a fallback comes into play as does CleanTalk's metrics.
I'm usually not much of one for analytics beyond what my server provides, but their organization and reporting of who is blocked and why is both intriguing and useful. I had a lot of people "talk up" how great it was, and I have to say that so far, I'm impressed.
... and at this point you guys know me, how often do I say that?
Anyhow, I'm going to give it a month to see how well it holds up. In particular I'm looking for false positives. Thankfully they report all blocks so I can easily go in and browse them.