Disable the WordPress REST API users endpoint on your child site.
Snippet Type
Execute on Child Sites
Snippet
add_filter( 'rest_endpoints', function( $endpoints ){
if ( isset( $endpoints['/wp/v2/users'] ) ) {
unset( $endpoints['/wp/v2/users'] );
}
if ( isset( $endpoints['/wp/v2/users/(?P<id>[\d]+)'] ) ) {
unset( $endpoints['/wp/v2/users/(?P<id>[\d]+)'] );
}
return $endpoints;
});