function plt_hide_mammoth_docx_converter_metaboxes() {
$screen = get_current_screen();
if ( !$screen ) {
return;
}
//Hide the "Mammoth .docx converter" meta box.
remove_meta_box('mammoth_add_post', $screen->id, 'advanced');
}
add_action('add_meta_boxes', 'plt_hide_mammoth_docx_converter_metaboxes', 20);