Which your Elgg version?

Thanks for fast response. I am using 3.1.1 the latest stable version

1.

In first, you can try to disable option for allowing users to set their own suggested privacy setting;

Administration -> Site settings (Content module)

Also, you can use the hook in your custom plugin.

start.php

elgg_register_plugin_hook_handler('view_vars', 'input/access', 'disable_access_levels', 999);

function disable_access_levels(\Elgg\Hook $hook) {
    $return = $hook->getValue();
    $return['disabled'] = true;
    return $return;
}

2.

Use this hook in your custom plugin (start.php):

elgg_unregister_plugin_hook_handler('view_vars', 'input/access', 'pages_write_access_vars');
elgg_register_plugin_hook_handler('view_vars', 'input/access', 'disable_pages_write_access', 999);

function disable_pages_write_access(\Elgg\Hook $hook) {
    $return = $hook->getValue();
    $return['value'] = ACCESS_PRIVATE;
    
    return $return;
}

Learn more about views.

Read this trick also.