Negative Keywords:

There is a negative keyword option in both Yahoo! and Google AdWords. Any
word that you do not want your ad to appear for can simply be blocked by placing -
badword beneath your keywords.

There are many words which would indicate a lack of desire to purchase anything.
Words like crack, free, pic, mp3 or download are often money losers if you do not
remember to include these in your - keyword list.

Example keyword list showing a negative keyword: (should be much longer and
more detailed, this is just an example though)
Eat Fruit
Buy Fruit
-free

Thus your ad would show up for many cheese queries, but for none that included
the words free. Google also allows you to filter negative keywords at a campaign
level.