Allow Only one Product

# Allow only one product
add_filter('woocommerce_add_cart_item_data', 'woo_custom_add_to_cart');
function woo_custom_add_to_cart($cart_item_data) {

    global $woocommerce;
    $woocommerce->cart->empty_cart();

    # Do nothing with the data and return
    return $cart_item_data;

}