Does any (old-time, trusted) user want to volunteer as the mod who goes back and deletes all the troll comments once a user has been designated a troll?

That seems like something that should be automated. I could give you a script that (for example) goes through a user page and deletes (or rather sends the appropriate button press that would be a delete command when run by a mod) any comment with less than 0 karma. It would make more sense for this to be an implemented feature of the site but "something I could implement in an hour without accessing the source code" seems like a suitable proxy for "is easy to make a feature of the site, one way or another".