functions.php

add_filter( 'woocommerce_dynamic_pricing_process_product_discounts', 'is_product_eligible', 10, 4 );
function is_product_eligible( $eligible, $product, $discounter_name, $discounter_object ) {
    remove_filter( 'woocommerce_dynamic_pricing_process_product_discounts', 'is_product_eligible', 10, 4 );
    
    if ( $product->is_on_sale() ) {
        $eligible = false;
    }
    
    add_filter( 'woocommerce_dynamic_pricing_process_product_discounts', 'is_product_eligible', 10, 4 );

    return $eligible;
}