Ban Countries!
Suggestions
Thoughts? ummm, hows this: The actual spammers are just renting a virtual server in india/poland/wherever and they actually live just down the road from you.
The reason they are logging in to your site is because some dude paid a $5 SEO service for 10,000 links and they're trying to sneak links into your site so they only have 9,999 left to post before they get their share of the $5.

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Whats the actual issue? is it the fake accounts existing that's troubling you, or the content they are posting?
Is the 'Spam Blocker' module not helping?
