BuddyPress.org

Create template overloads of these two pages:
buddypress\bp-templates\bp-legacy\buddypress\members\members-loop.php
buddypress\bp-templates\bp-legacy\buddypress\members\index.php

In the first, change
if ( bp_has_members( bp_ajax_querystring( 'members' ) ) ) :
to
if ( bp_has_members( bp_ajax_querystring( 'members' ) '&per_page=1000') ) :

In the second, delete the div that starts with:
<div class="item-list-tabs" id="subnav" role="navigation">