add_filter('woocommerce_package_rates', 'wf_hide_fedex_for_po_box_shipment', 10, 2);
function wf_hide_fedex_for_po_box_shipment($available_shipping_methods, $package){
    $shipping_method_to_hide = 'wf_fedex_woocommerce_shipping';
    global $woocommerce;
    $address  = ( !empty( $woocommerce->customer->get_shipping_address_1() ) ) ? $woocommerce->customer->get_shipping_address_1() : $woocommerce->customer->get_billing_address_1();
    $postcode = ( !empty( $woocommerce->customer->get_shipping_postcode() ) ) ? $woocommerce->customer->get_shipping_postcode() : $woocommerce->customer->get_billing_postcode();

    if ( strstr( $address, 'pobox' ) || strstr( $postcode, 'pobox' ) ) {
        foreach ($available_shipping_methods as $shipping_method => $value) {
            if( strpos( $shipping_method, $shipping_method_to_hide ) !== false ) {
                unset($available_shipping_methods[$shipping_method]);
            }
        }
    }
    return $available_shipping_methods;
}