You will have to create a template override for the mod_k2_content and remove it from the template's code.
To create the template override for the module just copy the Default folder from modules/mod_k2_content/tmpl/ to templates/your_template/html/mod_k2_content/
Then locate the code below and remove the link element or change the <a> element to <span>.
<?php if($params->get('itemTitle')): ?>
<a class="moduleItemTitle" href="<?php echo $item->link; ?>"><?php echo $item->title; ?></a>
<?php endif; ?>
