Spacebar Behavior

Tools - Global Options - Navigation
The space bar will open articles and allows you to page through the articles shown in the current display filter. The behavior of the spacebar when you reach the end of an article is governed by these settings.
The space bar behavior is set in "After single-key read:"
You probably want "View next unread article."
This way you can page through an entire newsgroup using only the spacebar, while skipping over previously read articles.

Notes:
There are settings for threaded and sorted views. In addition, there are separate tabs for offline and online use.
If you have loaded a large article and you don ’ t want to page all the way through it, use a keyboard navigation command, such as N (next unread), D (Down), or  U (Up) to move to the next. Or use the toolbar navigation buttons.