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This has happened to my site - 80 000 (that's about 97% of our total links) + forum account and blog comment spam with exact match text links.  Over 4000 domains.  It's simply not possible to get these links removed.  Most are abandoned blogs or forums that only spammers use.  Alot of them are also non english language sites. I did attempt to make contact with the webmasters of about 100 of these sites, and only got one response. Also as it's not a manual penalty, but an algorithmic penalty google say nothing can be done. The good news is many of the blogs have realized they have a security flaw allowing spam bots to create accounts and post comments and have subsequently deleted all spam comments or even shut the blogs down entirely. The negative SEO campaign continues though - new links are still being added.  I have seen some of my competitors targeted on the same forums / blogs too, a pretty clear sign it's a negative SEO attack. How would one go about discovering the source of the attacks?

Yeah, essentially what I'm doing is moving www.blog.com to www.othersite.com/blog in order to give othersite.com the blog's google juice.

In which case go for it if that's all that matters. User experience may be affected, depends on the designs, though as SEO is your only concern here, you're all set to press ahead

Actually, they're pretty easy to copy. Just a bunch of open blog posts and forum profile spam.