1. Create a new set of K2 template overrides inside your Joomla template: getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates

2. Edit the file default/category_item.php, locate the line <?php echo $this->item->introtext; ?> and replace with <?php echo $this->item->text; ?>. Done.

Actually, copy the file category_item.php to category_leading_item.php so you can only use it for your leading items. Then in category.php, replace the first instance of:

echo $this->loadTemplate('item');

with:

echo $this->loadTemplate('leading_item');

That should do the trick.