Disable out of stock variations

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);