Google can’t always work by itself and maintain the most spam-free search engine results in its search index. It’s thanks to the spammers who are making the heavy work and burden to google and so there is always seen some bad results in the Google search for many important keywords.
When we search for something in Google, it would just make our search experience worst when we see more spam results than some unique and good sites in the top.

So there is an option where you can report such kind of sites to Google. And by this, Google is made better and they work to penalize or deindex those kind of sites that disturb the normal static results in its search index.

You can report the site by an authenticated form or unauthenticated form, the difference being that the authenticated form is available in your Google webmaster tools only when you are logged into your Google account. And the unauthenticated form can be used by anyone to report a site, not needing to login to google account.

You can report a website if you are seeing one of the following reasons, as mentioned by the webmaster tools panel -

• Hidden text or links
• Misleading or repeated words
• Page does not match Google’s description
• Cloaked page
• Deceptive redirects
• Doorway pages
• Duplicate site or pages
• Or any other reason.

According to the Google spam report team -

We work hard to return the most relevant results for every search we conduct. To that end, we encourage site managers to make their content straightforward and easily understood by users and search engines alike. Unfortunately, not all websites have users’ best interests at heart. Violating our Webmaster Guidelines by means such as hidden text, deceptive cloaking or doorway pages compromises the quality of our results and degrades the search experience for everyone.

I have sometimes seen this working as i had reported a website which was a via**a page that suddenly popped into 7th position for a health keyword and i reported that one. It was removed in 4 days, and so i trust that one.

I advice to use the authenticated form to report a website, as during an update the google webmaster central had reported this -

Q: What happens to an authenticated spam report at Google?
A: An authenticated spam report is analyzed and then used for evaluating new spam-detecting algorithms, as well as to identify trends in webspam. Our goal is to detect all the sites engaging in similar manipulation attempts automatically in the future and to make sure our algorithms rank those sites appropriately. We don´t want to get into an inefficient game of cat and mouse with individual webmasters who have reached into the wrong bag of tricks.

Help google in becoming spam-free!