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This happened to me on the 3rd and I lost about 70% of my referrals, luckily the spammers were taken care of quickly and it came back the next day. This time the spammers aren't going away so quickly, it's been 3 days now and traffic is down horrendously.
I've sent dozens of spam reports to google and have tried contacting some hosting companies and advertisers to shut these down. One of the biggest offenders right now is using subdomain spam from the .cn domain (China) so the odds of me being able to stop them are pretty low.
The crappy thing is even if I get these spammers removed it can still happen again next month. There's got to be a solution.