editable_roles

add_filter('editable_roles', 'remove_higher_levels');

<?php
function remove_higher_levels($all_roles) {
    $user = gc_get_current_user();
    $next_level = 'level_' . ($user->user_level + 1);

    foreach ( $all_roles as $name => $role ) {
        if (isset($role['capabilities'][$next_level])) {
            unset($all_roles[$name]);
        }
    }

    return $all_roles;
}

add_filter('editable_roles', 'add_empty_editable_role');

<?php
function add_empty_editable_role($all_roles) {
    $screen = get_current_screen();

    if (! (isset($all_roles['']) || 'user-edit' == $screen->id)) {
        $all_roles[''] = array(
            'name' => __('— No role for this site —'),
            'capabilities' => array(),
            );
    }

    return $all_roles;
}