You can thank Google for enabling negative SEO. For years Google claimed that negative SEO was not possible, but they enabled it with their Panda and Penguin updates. All you have to do is focus on the issues that caused so many sites to get penalized and removed from Google search results.
In happened to me with a Paintball site. This was just prior to Panda and Penguin and while Google still claimed that negative SEO was not possible. I was aware of what was going on and watched it happen as a test to see if it affected the site's rankings. It took a couple of hundred posts in blog networks using unrelated contextual links with link text such as 'pig in a poke', 'mama said no', etc. Suddenly the site's rankings started to drop and eventually the site could not be found anywhere in the top 10 Google search results. This was a pristine site with no Google violations. Today, the blog networks are mostly gone, but the concepts behind negative SEO remain.
I know exactly who did it and how they did it. They did the same to my site and 5 or 6 others. Each blog post was focused on a paintball topic and contained one legitimate link to the culprit's site, along with 4 or 5 crappy links with unrelated text to competitor's sites. The culprit was an ecommerce site that wanted to remove higher ranking non-ecommerce sites from search results. The good news is that it looks like they got their own site removed from search results due to the use of the crappy blog networks.
My company no longer offers SEO services. We did get several requests for negative SEO services over the years but refused to engage in it.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin