I found a case study here: http://trafficplanet.com/topic/2369-case-study-negative-seo-results/ Where they managed to knock a website out of google for several keywords.
You guys think this is something new? This has been going on for years. It worked more on a KW level. Load tons of links to the KW and domain you wanted to sink and wait. After 2-3 weeks 30+ in the SERPS. *I've never done this but know people that have. #truestory
Yes, it can be. Just do black hat SEO in your competitor sites the so be it. But this is unethical way downing the competitor site. If you are really an effective SEO, just do your part as an SEO and let's see how effective you are in ranking your site in search engines.
This can be done if the competitor website is new, if the website is already old enough and has high authority this is almost impossible.
Negative SEO is nothing new by any means, it's just become more pronounced recently because main stream webmasters have become aware of it. Most people do not think about both ends of the spectrum... if you can move a site up with backlinks, you can also move it down.