Filter out items that contain a keyword

Is there a way to filter out all the feed items that contain a keyword. Basically can I negate the "Keyword filter" plugin.

For example, I don't want to import any tweets that are retweeted. Some folks still add "RT " in their tweets. So I wanted to use Feeds Tamper to exclude all tweets that contained the string "RT ".

Did I miss something or there is no negation function for the filter plugins yet?

Thanks!