Disable BuddyPress Signups feature.

bp-custom.php

<?php

function disable_bp_signups() {
	add_filter( 'bp_get_signup_allowed', '__return_false' );
	remove_filter( 'register_url', 'bp_get_signup_page' );
	remove_action( 'bp_init', 'bp_core_wpsignup_redirect' );
	remove_filter( 'wp_signup_location', 'bp_blogs_creation_location' );
	remove_filter( 'wpmu_signup_user_notification', 'bp_core_activation_signup_user_notification', 1 );
}
add_action( 'bp_include', 'disable_bp_signups' );