Referrer Spam Blocking
Referer spam is where someone will hit your site with the referer section of the browser request set to be a site they want to drive traffic to. The idea is to have links to those sites how up in any public stats pages you may have - which are common for blogs and forums.
With NGINX you can use the valid_referers module to handle this:
Where you replace the badreferer domains with the referer domains that you wish to block.