As for every open blog (open as in everyone can post comment without register first), anti-spam is of high priority.
With further investigation, I decided to try out Captcha! (BTW: there is an open contest sponsored by eZ among others, in which one of the three available categories of application is captcha. However, as it turned out that it has too little information in the English post, I do not know much about this contest)
According to the document, I have to find a fancy font to use. I followed the suggestion to dig in website dafont. ” 28 Days Later” seems to be a decent while fancy one, so I downloaded it and put it in the same dir as the Captcha! plugin.
It is also necessary to create a web server writable dir for Captcha! to put the generated image files and the dir has to be under document root.