A thread at Google Webmaster Help asks a question that goes unanswered for the most part, "Negative SEO. How can we let the Webmaster Team know?"
How do you let Google know when you think you've been hit by negative SEO?
Good question, I believe this was answered by various Googlers.
Besides for most of them saying that it is rare to be hurt by negative SEO, if you are worried, you can post the details in Google Webmaster Help and also even submit a reconsideration request, if you have a manual action, and let them know that way.
Truth is, either way, the best response you'll get from Google or someone trying to help you with your Google issues but do not work from Google, is the Google Webmaster Help forums.
You can also try to get on a live hangout with John Mueller or another Googler by visiting Google Webmaster on Google+ and finding one of their live hangouts.
Either way, you will almost have to always do work to "fix" the issue even if someone else did it to you. To get help on those steps, you need to isolate what your issue is and then fix it from there.
Forum discussion at Google Webmaster Help.
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