You will need to install a user script manager extension to install this script.
To ignore a topic or forum, hover over it and click on its trash icon (all ignore controls are displayed in screenshots below for demonstration purposes).
To unignore a topic, select "Toggle Ignored Topic Display" in the context menu (right-click menu for Greasemonkey, or look in the script manager extension's menu for other script managers). Ignored topics will appear and be highlighted in red - click on their trash icon to unignore them.
New (temporary) feature: this script now also makes topic links go to the first new post while we're waiting for that feature to get re-added to the forum.