You need to modify this file
components/com_k2/models/item.php

Remove the following validation code from around line 629

if ($user->guest){

$db = & JFactory::getDBO();

$query = "SELECT COUNT(*) FROM #__users WHERE name=".$db->Quote($userName)." OR email=".$db->Quote($commentEmail);

$db->setQuery($query);

$result = $db->loadresult();

if ($result>0){

echo JText::_('The name or email address you typed is already in use!');

$mainframe->close();

}

}