Take a company you have a problem with
1. Imagine you hate Ryanair. I do not btw they are just an example of a company this would work on.
2. You decide you want to harm them commercially
3. You go to their terms and conditions here and see they do not allow linking to their website
"Links to this website. You may not establish and/or operate links to this website without the prior written consent of Ryanair. Such consent may be withdrawn at any time at Ryanair’s own discretion."
There is a list of other companies that do not allow linking listed here. That where I heard of this T&C clause.
4. Now spider all the websites that do link to them. This is easy to do and you now should have a list of many websites. Either from a whois or just from scraping the website get an email address for each site.
5. Send all these linking websites official looking notices that remind them that "You may not establish and/or operate links to this website without the prior written consent of Ryanair" including a link to the official Ryanair terms and conditions. You would have to be careful here. You would not claim to be Ryanair for legal reasons. You do not want to break any laws or threaten anything. Just say you are from some official sounding company "John Smith and associates business compliance coordinators" and remind them of what the companies own policies say. It is not like you are lying.
6. Watch Ryanair drop down search rankings as people delete links to them. Search rankings are mainly based on how many (and what sort of) sites link to you. As they drop down the search engine rankings they will suffer commercially. With my google of "cheap flight dublin london" Ryanair comes up as the third site. Which implies they must get a fair amount of traffic from search engines.
I think this attack should be called Search Engine Deoptimization.