To remove choices from a catalog item field only in an Order Guide, create a Catalog Client Script (onLoad) in the Order Guide context:

if (g_form.getParameter('sysparm_guide') != '') {
g_form.removeOption('field_name', 'value1');
g_form.removeOption('field_name', 'value2');
}

This hides the choices only in the Order Guide without affecting the original catalog item.