Just click on START next to "Ignore list."
Be advised that there may be some delay before this selection takes effect.
NOTE: If you use the "ignore" feature with an anonymous poster, you will have ignored ALL anonymous posts on the site from that time forward. (Anonymous posts predating your choice will still appear.) To reverse this selection, visit this link:
Anon ignore toggle
To UN-IGNORE that user, just return to the page, and the button will have changed to "Stop." Click on it, and you're all set.
You can see a list of users that you have placed in "ignore" status by clicking on the "buddy" list represented by the or by using your "My" menu.
Also, when you view a thread that contains a post by a user you have ignored, you will see this in their post: