It's actually extremely difficult to harm a website with negative seo if the website itself has a healthy link profile to start with. If your site is full of spammy backlinks well then it's easy peasy. Having said that, Google's tone has been changing over the last few months with comments like "you can't harm a website with negative SEO" back in September 2011 to "Google is working hard to prevent negative SEO links from affecting sites" in June 2012.
Hi Brian,
This isn't true. Its a lovely sentiment, and yes Google are very, very smart. But this isn't true. It's not hard at all.
I have a site that got a penalty (one of my own) last year in September. I was shocked. It turned out that 5 business friends of mine had links to a site I own on every single page. They thought they were doing me a favour. The problem is that it was done years ago - some as long ago as 2007 and 2008. In that time, some have emigrated, sold their businesses or we've fallen out of touch. Just 5 domains was enough to penalise a 10 yo PR6 site with 16k inbound links (naturally gained due to nature of the business)
It took 5 months to get a revoke. My last recon request stated, simply: "This is bull****, I have no control over the creation of these links, the scale, size of the sites nor their value. I cannot force people who've acquired a site as a result of a business acquisition to remove something. Some of these date back 5 years. Do what you want..."
It worked. I'm guessing you can't do that every day.
Recently an ex-client got penalised. An ad they ran on a website for years got republished without a nofollow tag - they were dumped in the rankings within days.
Negative SEO is not only real, its really, really easy.