How to Remove the Custom Theme Panel from a Theme?

The three easiest options:

1. Limit users’ user role to Author or below
2. Change the $cap parameter from 'edit_theme_options' to 'edit_options' in the add_theme_page() call that adds the settings page

3. Remove the settings page entirely, by removing the callback that adds the settings page. e.g. if the Theme uses:

   function theme_slug_add_settings_page() {
       add_theme_page( $args );
   }
   add_action( 'admin_menu', 'theme_slug_add_settings_page' );
   

   Then you would remove that via:

   remove_action( 'admin_menu', 'theme_slug_add_settings_page' );