(since 2.5)
By default, full/single posts have a "Go back to the previous page" link under the post contents. It's a javascript link that takes you back to the page where you came from (which could be one in your site, or any other external page).
If you can change or remove this link, create/edit component nm-init and insert this (enclosed between <?php and ?> tags):
Disable "Go back" link:
nm_set_option('gobacklink',0);
Replace it by a link to your main news page:
nm_set_option('gobacklink','main');
You can also change the text in the link. Example:
nm_set_text('GO_BACK','Back to main page');