The low quality comment spamming is interesting. I'm not personally tempted to do it.. whether out of honor or it just isn't practical anyways to be a troll longterm.. but it definitely seems like a viable angle that should have some sort of counter-measure.
Throwing your flag is one way, but it seems imperfect because it isn't that there's something flag-worthy in that particular post. And plus you don't necessarily want to take one for the team and use your curating on stuff like that.
I think the reputation feature needs to expand. I'm actually not really sure at all how it works currently. I've thought that there should be some way to rate "quality" of posts (I guess I mean comments more than blog posts, but either). Like whether or not you want to use your curation power on a post, you can select how good you think it is.
If it's a 1-10 sort of thing, the algorithm should probably factor in the rater's tendencies and how often they select 10 (or else the game theory is to always pick 1 or 10 to maximize your influence). Even a 'constructive' vs. 'useless' would address what you're talking about .. 1-10 would do more to sort out excellence.
I guess a less complicated idea is additional flags, for comment spam and maybe also for plagiarism. And it just shows up next to that person's name for a certain amount of time or whatever when they're flagged by enough people for it.