You need to hook into acf/save_post before ACF saves values, so with a priority of < 10
add_action('acf/save_post', 'YOUR_FUNCTION', 1);
Then you need to access the values in $_POST['acf'] because the fields have not been saved yet. Once you are done then you can unset $_POST['acf'] and this will cause ACF to not save anything.
Doing any of this after ACF has saved the values will be to late.
