Enable Comment Moderation
Comments are a great way to increase effective conversation and build loyal followers. But, in order to stop spammy agents from commenting in your valued site, it is necessary to enable comment moderation. Additionally you may also stop the user from adding hyperlinks in the comments by changing the configuration and removing HTML from it.
Use Captcha
Captcha is a great way to restrict bot spam. Consider using Google’s free reCAPTCHA’s service.
Use rel=nofollow tag
In order to save your site from the negative action taken by Google, use rel=nofollow tag which stops the hyperlink from getting the precious link juice that is used to calculate PageRank.
Use robots.txt
Another way is to use robots.txt and disallow the folder containing the spam from getting accessed by the search engine bots.
Example:-
user agent: *
Disallow: /comments/
Also See:-
Advertorial Link Building Spam
Google Patent to Identify Spam in Information Collected from a Source
Rel=author an Important Ranking Signal for Google
A Deeper Understanding of Google Knowledge Graph
Latent Semantic Indexing
Facebook Graph Search Optimization
Getting Listed on Search Engines
Universal Analytics
Learn Seo Step by Step
Google Tag Manager
Google Sandbox effect
