Article Archive Flag, Set to yes and the article is removed from all lists on the system. This is preferable to deleting the article as it is still accessible by anyone who has bookmarked it and by search engines that may have indexed the page.

Once an article is obsolete and/or should be removed - set this to ´Yes´

There is also a preference setting that will automatically archive articles based on their age. This can be configured/overridden on a per-category basis as well.