Spam is the biggest threat to organic search engines, whose goal is to provide quality search results for keywords or phrases entered by their users. Google's PageRank algorithm update ("BigDaddy") in February 2006—the final stage of Google's major update ("Jagger") that began in mid-summer 2005—specifically targeted spamdexing with great success. This update thus enabled Google to remove a large amount of mostly computer-generated duplicate content from its index.
You should also make sure you aren't competing with your own affiliates for eyeballs. Any marketing channels you're using, such as search engines, content sites or e-mail lists, should be off limits to your affiliates. Put marketing restrictions into your affiliate agreement and notify partners immediately. It's your program--you set the rules. Or, if you prefer, you can let your affiliates run the majority of your internet marketing.