"PBH has run for over 10 years as a great place to get and share information about Colombia. Unfortunately, during the last few years, there has been a (very) small minority of people who thought it was ok to personally attack, threaten and harass, both on phb, on other websites and in real life, other members that they disagree with. You know who you are. "
It sounds like a different problem than just throwing up a captcha. Have you banned the users/IPs of the offending people? Can't you just add a "report user" button by each post and then have users or IPs with X number of reports automatically banned?