A Search Engine Watch Forums thread has discussion about a service one member was offered.
In short, the service is composed of two offerings:
(1) Damage your competitor's search engine rankings (2) Protect your own search engine rankings
They use threats in their email marketing message, such as "Pay up or have your forum spammed!" and "Your forum will be spammed in the next few days" and then "Pay up to this url or have your forum heavily spam."
What should you do if you get such an email? Forward it to Google or let me know.
But seriously, all you need to do is "just hit the delete button," as forum administrator, Robert Kerry said.
What can I do if I'm afraid my competitor is harming my ranking in Google?
There's almost nothing a competitor can do to harm your ranking or have your site removed from our index. If you're concerned about another site linking to yours, we suggest contacting the webmaster of the site in question. Google aggregates and organizes information published on the web; we don't control the content of these pages.
"Almost nothing" are the words used here, so technically, it is possible.
Forum discussion at Search Engine Watch Forums.