This method works for Kunena 3.x and later versions. It is always recommended to give credits to the developer. Follow the steps if it is necessary for you to remove Powered By Kunena Forum Footer Link.
Open the file from your Joomla installation folder: /libraries/kunena/view.php
Find the following code (use ctrl + F)
Change it to:
// echo $credits;
Save the view.php file. Now, the footer should have gone!
The code prints out forum footer. You are commenting the code line so that it becomes inactive.