OK – you can try this but be warned – not tested out that much so you might have to undo it again!
First up edit the core file /simple-press/forum/feeds/sp-feeds.php
Locate the comment that reads ‘# Execute the query’ and immediately ABOVE that line add this:
$where = apply_filters('sph_rss_where', $where, $feed);
Then open up the spFunctions.php file of your SP theme – in the templates folder. And add this function:
add_filter('sph_rss_where', 'rssFirstPostOnly', 1, 2);
function rssFirstPostOnly($where, $feed) {
$where.= ' AND '.SFPOSTS.'.post_index = 1';
return $where;
}
Be careful with all the full-stops and spaces in there. It needs to be exact!
See if this does the trick. If it does we can add that filter to the RSS feeds code so you know it will be there in the next update of SP.
As always we recommend that you create your own SP theme so that any customisation is not lost during a future update. (http://codex.simple-press.com/codex/themes/theme-basics/creating-a-theme/)