If you use Kunena forum extension for Joomla, there is a simple way to remove the footer back link. The following method works for Kunena 5.x and later versions. Follow the steps if it is necessary for you to remove Powered By Kunena Forum Footer Link. This works as of Kunena version 5.0.14.
Open the file from your Joomla installation folder:
Find the following code (use ctrl + F)
Change the above code to:
Save the display.php file.
Now, the Kunena footer should have been removed!