this is google patent.but i dont undestand http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.htm&r=1&p=1&f=G&l=50&d=PTXT&S1=7,509,344.PN.&OS=pn/7,509,344&RS=PN/7,509,344
You won't be hurt by someone spamming to your website, if that were the case all you would have to do is take your top competition and spam links to their website in order to take them down. However, I do believe that Google evaluates the overall quality of links to your website as a whole, and if 100% of them are from sources they consider spam, the net effect is the links do not benefit you (yet still do not hurt you). However if the bulk of links to your website are from trusted, high pagerank and high authority sources, spam links would even be somewhat beneficial. I've had personal experience with websites that were 100% spam linked, 50% spam and 50% quality linked, and 100% quality linked. You're best off going the 100% quality route, but if you end up getting spammed to, that won't hurt you as long as overall your backlinks are quality.
Spam links rarely pass any PR, either the website allowing spam is either considered a "bad neighborhood" and the outgoing links have no credit, or the spammer is sharing the outgoing link count with 100 other spammers and whatever pagerank they divide between the other 100 links is virtually nothing. Spam links don't pass authority. An authority link is a link coming to your website from a related website, i.e. if your site is about cars, you have an incoming link from a website that is primarily about cars. Spammers typically post all kinds of text along with their links that change the content of the page to rank for whatever they're trying to rank for and as a result, the whole theme of the website is scattered all over the place, thus you don't get credit for incoming links from a site with the same content as your own. Defining spam, its just posting links to your own website without offering the website you're posting on any content in return, i.e. a thoughtful post that reflects upon the topic of the article or blog being posted on. You are far better off spending an additional 5 minutes per link and post a quality, thoughtful comment on their blog or article post. These places allow you to link back to your website (99% of them give you a spot to advertise yourself for free, for putting in the effort to write a quality post). If you find high quality blogs, articles and forums that do not allow spam, the incoming link you receive is infinitely more valuable. I've extensively tried both methods and competed against sites using both methods, quality links are far superior for getting high in google SERPS.