[kh99]

If you are talking about the feed from your own site, you could edit external.php and find the 2 lines like this:

$xml->add_tag('description', fetch_trimmed_title($plainmessage, $vbulletin->options['threadpreview']));

and replace $vbulletin->options['threadpreview'] with 0.

You could also use a plugin on hook external_start, and code like:

$vbulletin->options['threadpreview'] = 0;

Edit: also note that the feed is cached, so you might not see the changes right away.