Each email address in the Trusted Sender section has a "Delete" button next to it. To remove an email address from your Trusted Sender list, just click the "Delete" button next to the email address you wish to remove.
If you've gotten a message that you think is spam, and its headers indicate it was whitelisted, go ahead and remove that address from your Trusted Sender list. However, we do not recommend that you remove Trusted Senders just because you don't recognize them.