BuddyPress.org

At least two ways – assuming you’re using child theme of bp-default:
1. in style.css only
body.registration #sidebar { display: none; }
body.registration div#content div.padder { margin-right: 0px; border-right: none; }

2. Template change plus one line in style.css
copy register.php from bp-default/registration/register.php to your child-theme-folder/registration/register.php
replace div class=”padder” with div class=”padderwide”
delete line [?php locate_template( array( ‘sidebar.php’ ), true ) ?]

in style.css add
.padderwide { padding: 20px; }