Just click on the user's name (above their avatar) on the left and select View Public Profile. That takes you to their profile page. In the aqua bar underneath the username and avatar, you will see a link that says Add <user> to Your Ignore List (third option, all the way to the right). Click on that link. Next you will see a confirmation page. Click Submit and your done.
If you ever want to remove someone from your Ignore List, just go to your User CP (at the top left of any page) and click on the "Buddy/Ignore List" link on the bottom left (under Miscellaneous).