I used to have a text document on my desktop containing a list of domains that contained autogenerated content, each with a minus in front, like:

-stupidautogeneratedcontent1.com -stupidautogeneratedcontent2.com etc

I figured sooner or later Google would pick up the signal but I think instead they just started ignoring my "- requests" as I stopped using them. edit: or maybe they fix the problem. Spam sites used to be a problem during the early decline of Google. I think what happened was that problem actually almost disappeared for me and was replaced by irrelevant results from non-spam-sites

Edit: mahalo.com was one of those, https://en.m.wikipedia.org/wiki/Mahalo.com