Also your Daredevil friend is trying to tell me to give Kilgrave a second chance because maybe he really is changing and um I said I understand how HE might think so but pardon my candor: fuck that. :)
ALSO also I researched permanent vocal cord damage and basically we need to infect him with laryngitis and not let him heal, burn his throat enough that his cords damage (chronic, relentless acid reflux also works) or just slice his throat which would probably kill him before harming his voice. Oh also we can paralyze his vocal cord nerves but that would also need to be via surgery. So maybe just keep breaking his windpipe.