If you look up in the NavBar, directly under the breadcrumb navigation, you will see a menu link - User CP. Click on that link.
When the new page opens, look in the Control Panel, in the left-hand column. Scroll to the bottom, to the Miscellaneous section and click on Buddy/Ignore Lists.
In the center pane, under Ignore List, there is an text entry window, Add New User to List. Add the username you wish to ignore and click on Save List.
You're finished.