You have to edit one of the core files of WordPress to get rid of the WordPress link in the widget box or you have to use any of the custom meta widget plugins which are available.
Use the core method:
Go to /wp-includes/ and edit default-widgets.php
Search for „wordpress.org“ and you find the corresponding line. You have to comment out the complete line like this:
<?php /*echo apply_filters( 'widget_meta_poweredby', sprintf( '<li><a href="%s" title="%s">%s</a></li>', esc_url( __( 'http://wordpress.org/' ) ), esc_attr__( 'Powered by WordPress, state-of-the-art semantic personal publishing platform.' ), _x( 'WordPress.org', 'meta widget link text' ) ) ); */?>
Everytime you update your WordPress installation you have to repeat this step as WordPress overwrites your changes, so its recommend to use the second method:
Use the plugin method:
Get a custom widget plugin here: