Somewhere out there is a forum that allows posting either without an account or has a registration without a
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The bots that post spam to blog or forum posts find that site and post spammy comments to it. That site then tracks the IP Address, email address, username that the bot used when posting the comment. By providing that information as a list, site owners are able to configure their forums to block posts from those users.
There is a similar concept that server administrators use for email spam called a "Spamtrap"