Google and other search engines allow you to report keywords for which your ads should not be shown. If you’re paying for “broad match” keywords, you’ll see a lot of web site traffic from people who have typed your keyword with the word “free”, “stock”, or “jobs”. Do you want to pay money for visitors who’re looking to pay $0 for what you sell? Do you want to pay money for visitors who are researching stock info on what you sell? Be sure to use that “negative keyword folder” to get rid of these worthless clicks.