CB doesn't probably have this problem because it is Jefe's proprietary software. And the spammers aren't looking for the lone proprietary site. They want to be able to attack all phpbb sites at once, or all the vbulletin sites using an automated hacking program.
I could , for example, ban registration with all common free email hosts (gmail, yahoo, msn, homtail, etc.). But that would be a pain for everyone.
I could also moderate all new signups, but that is a pain as well.
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