Add this code to function.php file
/**
* Disable out of stock variations
* https://github.com/woocommerce/woocommerce/blob/826af31e1e3b6e8e5fc3c1004cc517c5c5ec25b1/includes/class-wc-product-variation.php
* @return Boolean
*/
function wcbv_variation_is_active($active, $variation)
{
if (!$variation->is_in_stock()) {
return false;
}
return $active;
}
add_filter('woocommerce_variation_is_active', 'wcbv_variation_is_active', 10, 2);