Bilbo,
CAPTCHA is the image verification used before a user signs up or posts as anonymous. The new version of Threads now has this built in. I'd update to it if possible for you. The new version has many new characteristics that make it worth the expense and effort. Forums run faster and safer along with many new forum owner requested features.
I just took a look at your site. With 10,000 or less posts, an import to the version 7 series should be easy to do.
