How to report Search Engine Spam:

Since spamming practices are constantly evolving, it is important to know what the major search engines specifically say about spam and what practices are definitely not allowed if you would like to rank in top-tier search engines. Plus, every ethical SEO should know how to properly report any spam that they see so the search engines can correct their algorithm accordingly.
How Google Defines Spam

As part of their Webmaster Guidelines, Google outlines techniques to use to help Google locate, index and rank your website. They also specificially state that the following techniques may lead them to remove your site from the Google index:
Hidden text or hidden links.

Cloaking or sneaky redirects.
Automated queries to Google.
Pages loaded with irrelevant keywords.
Multiple pages, subdomains, or domains with substantially duplicate content.
"Doorway" pages created just for search engines, or other "cookie cutter" approaches such as affiliate programs with little or no original content.

However you should keep in mind that these aren't the only practices that Google disapproves of. Generally, Google doesn't like their results manipulated by deceptive practices. Their recommendation for webmasters is:

Webmasters who spend their energies upholding the spirit of the basic principles listed above will provide a much better user experience and subsequently enjoy better ranking than those who spend their time looking for loopholes they can exploit.

To combat common search engine spam practices employed by rogue SEOs, Google has also posted a list of practices that should raise a red flag when you are looking for a search engine optimizer. According to Google, feel free to walk away from an SEO who:

owns shadow domains
puts links to their other clients on doorway pages
offers to sell keywords in the address bar
doesn't distinguish between actual search results and ads that appear in search results
guarantees ranking, but only on obscure, long keyword phrases you would get anyway
operates with multiple aliases or falsified WHOIS info
gets traffic from "fake" search engines, spyware, or scumware
has had domains removed from Google's index or is not itself listed in Google
How to Report Spam to Google

Google has a form that allows you to report spam to Google or you can e-mail Google at spamreport@google.com. Note that Google rarely manually removes websites from the engine. Instead, it tweaks the search engine algorithm and spam detection software to try and eliminate the spam technique that is clogging up the engines.

How Yahoo! Defines Spam
NOTE: Altavista, All the Web and Inktomi are all owned by Yahoo!, so the Yahoo! spam policies and webmaster guidelines also apply to these search engines.

According to Yahoo!, search engine spam is webpages “that are considered unwanted and appear in search results with the intent to deceive or attract clicks, with little regard for relevance or overall quality of the user experience.” Officially, Yahoo! does not want to index sites with:

Text that is hidden from the user
Misuse of competitor names/products
Pages that have substantially the same content as other pages
Multiple sites offering the same content
Pages in great quantity, which are automatically generated or of little value
Pages dedicated to redirecting the user to another page
Pages that give the search engine different content than what the end-user sees
Pages built primarily for search engines
Pages that use excessive pop-ups, interfering with user navigation
Pages that use methods to artificially inflate search engine ranking
Sites with numerous, unnecessary virtual hostnames
Excessive cross-linking with sites to inflate a site's apparent popularity
Pages that harm the accuracy, diversity, or relevance of search results
Pages that seem deceptive, fraudulent, or provide a poor user experience
How to Report Spam to Yahoo!

If you find a site that is spamming in Yahoo!, you can report the spam through a form on their website.

NOTE: In addition to reporting spam, you can also report copyright violations to Yahoo!. To request that they remove any content published in violation of copyright protection, e-mail them at copyright@yahoo-inc.com.

How Teoma / Ask Jeeves Defines Spam
One of the most definitive sources of the Teoma / Ask Jeeves spam policy is on their Site Submission Terms page. Among the techniques that will keep you from being ranked are:
Having deceptive text
Having duplicate content
Having metadata that does not accurately describe the content of a web page
Including off-topic or excessive keywords
Fabricating pages to lead users to other web pages
Showing different content than the spidered pages to users
Using intentionally misleading links
Using self linking referencing patterns
Misusing affiliate or referral programs

How to Report Spam to Teoma / Ask Jeeves
To report search engine spam to Ask Jeeves or Teoma, e-mail them at jeeves@askjeeves.com
How MSN Defines Spam
MSN Search has recently added content guidelines to their website, explicitly stating that the MSNBot will see the following techniques as search engine spam:
Stuffing pages with irrelevant keywords in order to increase a page’s keyword density, including ALT tag stuffing.
Using hidden text or links.
Using techniques such as creating link farms to artificially increase the number of links to your page.

Also, in an e–mail announcing the second preview release of the new MSN search, Microsoft mentioned cloaking and having duplicate content on multiple domains as things that will lead your site to being penalized or removed from the MSN Search index.
How to Report Spam to MSN
To report search engine spam to MSN, use the form on their website.
Have you seen any search engine spam lately? Instead of submitting spam reports to each engine, you can also simply submit a spam report through SEOToolSetTM.

Even those who are spamming right now and think they are getting away with it, should keep one thing in mind, when competitors check out your site (and they do), they will see it is spam and they may choose to report you. Once you have been reported to a search engine, you are likely to be penalized in search engine results for using your spam technique.