I’m not familiar with the pinboard theme, but generally there are a few ways for doing this.
one (recommended), create a child theme of pinbboard, and add a custom footer.php (assuming) in the child, with the text removed.
two (recommended), create a child theme of pinboard, add a filter in the functions.php file to remove or alter the text – if Pinboard supports this, I don’t know)
three, (not recommended unless options 1&2 are not possible), edit the footer.php file in Pinboard, and remove the text.
You have an option for that in Theme Options.
It’s typically here:
Goto “Appearance > Editor”
Click on the “Footer.php” link
“Ctrl + F” and search for “Powered by”
Comment out all the lines containing those words (Use “//” to comment out)
For example:
Original: <?php echo sprintf( __( ‘Powered by %1$s’, ‘pinboard’ ), $wordpress_credit_link ); ?>
Modify it to: <?php // echo sprintf( __( ‘Powered by %1$s’, ‘pinboard’ ), $wordpress_credit_link ); ?>