So as the title asks how would one go about reporting a site for keyword stuffing to google? I can't seem to find anyway to do this other than reporting a site for being "spammy".
According to Google:
"Keyword stuffing" refers to the practice of loading a webpage with keywords or numbers in an attempt to manipulate a site's ranking in Google search results. Often these keywords appear in a list or group, or out of context (not as natural prose). Filling pages with keywords or numbers results in a negative user experience, and can harm your site's ranking. Focus on creating useful, information-rich content that uses keywords appropriately and in context.
Based on this information, I would presume that Google proactively takes action against these pages without need for reporting the site.
If you feel like the website still needs to be reported, you can do so here: