UPDATE:
Tip received via James

THE new Kunena

// Credits
echo ' ' . CKunenaLink::GetTeamCreditsLink ( $catid, JText::_('COM_KUNENA_POWEREDBY') ) . ' ' . CKunenaLink::GetCreditsLink ();
if ($this->params->get('templatebyText') !=''):
echo ' :: params->get('templatebyLink').'" rel="follow">' . $this->params->get('templatebyText') .' '. $this->params->get('templatebyName') ? $this->params->get('templatebyName') : '' .'';
endif;
echo '';

// display footer


Just change to

// Credits



// display footer



BELOW IS THE PREVIOUS CONTENT OF THIS PAGE

The code is GPL so you can do whatever you want with it.

But do forward the Kunena team a donation if you can!


The relevant code is in the kunena.php file in components/com_kunena/

Change

echo '<div class="fb_credits"> ' . CKunenaLink:: GetTeamCreditsLink($catid, _KUNENA_POWEREDBY) . ' ' . CKunenaLink:: GetCreditsLink();

to

    echo '<div class="fb_credits"> ';

There are about four other files with the "Powered by Kunena" link -- rss and mail stuff. You can simply search the folders for those.