Referrer spam is a technique involves making repeated web site requests using a fake referer URL to the site the spammer wishes to advertise. Now a days I am getting to much referrer spam on 99webtools that causes inaccurate stats at Google analytics. You can also find some of these referrer in your analytic. Here is the list of referrer that spamming 99webtools.
floating-share-buttons.com 4webmasters.org trafficmonetizer.org erot.co yourserverisdown.com event-tracking.com traffic2money.com buttons-for-website.com success-seo.com free-social-buttons.com Get-Free-Traffic-Now.com e-buyeasy.com videos-for-your-business.com callgirlservices.in
To block referrer spam add following code in your .htaccess file
RewriteCond %{HTTP_REFERER} floating-share-buttons.com [NC,OR]
RewriteCond %{HTTP_REFERER} 4webmasters.org [NC,OR]
RewriteCond %{HTTP_REFERER} trafficmonetizer.org [NC,OR]
RewriteCond %{HTTP_REFERER} erot.co [NC,OR]
RewriteCond %{HTTP_REFERER} yourserverisdown.com [NC,OR]
RewriteCond %{HTTP_REFERER} event-tracking.com [NC,OR]
RewriteCond %{HTTP_REFERER} traffic2money.com [NC,OR]
RewriteCond %{HTTP_REFERER} buttons-for-website.com [NC,OR]
RewriteCond %{HTTP_REFERER} success-seo.com [NC,OR]
RewriteCond %{HTTP_REFERER} free-social-buttons.com [NC,OR]
RewriteCond %{HTTP_REFERER} Get-Free-Traffic-Now.com [NC,OR]
RewriteCond %{HTTP_REFERER} e-buyeasy.com [NC,OR]
RewriteCond %{HTTP_REFERER} videos-for-your-business.com [NC,OR]
RewriteCond %{HTTP_REFERER} callgirlservices.in [NC]
RewriteRule ^(.*)$ – [F,L]
To get full list of referrer involving in spam See this https://github.com/piwik/referrer-spam-blacklist