The code you’re looking for is in the file footer.php
<footer id="colophon" role="contentinfo">
<div class="site-info">
<?php do_action( 'twentytwelve_credits' ); ?>
<a href="<?php echo esc_url( __( 'https://bitnami.com/stack/wordpress/', 'twentytwelve' ) ); ?>" title="<?php esc_attr_e( 'Semantic Personal Publishing Platform', 'twentytwelve' ); ?>"><?php printf( __( 'Proudly powered by %s', 'twentytwelve' ), 'BitNami WordPress Stack' ); ?></a>
</div><!-- .site-info -->
</footer><!-- #colophon -->
Find that code and delete it. The only thing is that if you haven’t made a child theme yet then if you update you’re theme in the future then it will just come back.