Yes, you can block retweets. To change this setting, the easiest way to get there is to log into your account and on the Overview page, click on the "Monitoring # Terms" link in the Content Sources section next to where it says Twitter. That will bring up the settings specifically for Twitter, and you'll find the option there.
The other way to get there would be to click the Edit button on any page when inside your wall, then hit next (or click the Wall Content tab up top), and then on the Content screen of the builder in the Twitter row, click on the edit icon (it looks like a pencil).
Since retweets are specific to Twitter, they are located in Twitter's options.