Force add to cart quantity to 1 and disable +- quantity input

functions.php

/**
 * @snippet       WooCommerce Display Separate Cart Items for Product Quantity > 1
 * @author        Carla Chalmers
 * @testedwith    WooCommerce 3.5.1
 */

// -------------------
// Split product quantities into multiple cart items
// Note: this is not retroactive - empty cart before testing

function split_product_individual_cart_items( $cart_item_data, $product_id ) {
	$unique_cart_item_key         = uniqid();
	$cart_item_data['unique_key'] = $unique_cart_item_key;

	return $cart_item_data;
}

add_filter( 'woocommerce_add_cart_item_data', 'split_product_individual_cart_items', 10, 2 );

// -------------------
// Force add to cart quantity to 1 and disable +- quantity input
// Note: product can still be added multiple times to cart

add_filter( 'woocommerce_is_sold_individually', '__return_true' );

//remove message CONTINUE SHOPPING after an item has been added to cart
function filter_wc_add_to_cart_message_html( $message, $products ) {
	return "<span>".$message."</span>";
};
add_filter( 'wc_add_to_cart_message_html', 'filter_wc_add_to_cart_message_html', 10, 2 );