This request is twofold. I'm not sure they're both needed, maybe one precludes the other.
1. Remove comments from posts with the #spam tag from global chatter. The majority of such posts are truly spam so there's probably not a huge side-effect to this. Several of the recent spam accounts have started commenting on their own posts to get greater visibility, and even once the post is tagged and thus filtered from global posts, the comments are still visible in global chatter.
2. Comment tagging - or at least the ability to mark comments as spam. This feature, coupled with the option to filter spam comments may be a better option but both features could conceivably work together.