When you see a post by a person you wish to add to your ignore list, just click on their name next to the avatar to navigate to their profile page. Once there you'll find an Ignore User button under their avatar. Click it and POOF!
To reverse it, just do the same thing except click the Unignore User button, or go to your own profile, click on the My Ignored User list button and unignore people on the list.