Its not so much of the forum software, but the powerful software available to spammers and crackers. With the cashflow available to these people who do this stuff there is a large array of applications that can register and "participate" on about any forum.

These applications can read captcha images, fill out only "required" info (by reading the page) and even "test posting" their spam message is specific forums (by reading the page). As well as other configruable options.

However, a simple text string or image asking the user to solve a simple logical problem will stop these things in their tracks (at least for now)...

Heres the preliminary anti-bot captcha system I'm working on until I get the "real deal" coded.
http://mgcjerry.net/captcha.html