As for the external posts getting hit with BS posts, I think lessismore is right, just "ignore" someone and they will no longer show up. The only thing I see as a potential problem for the future are people commenting. How to ignore a user that comments on everything but has nothing to say or someone that is annoying or a blatant troll? How to avoid that is the question. Can you block someone from posting a comment on anything you post? Maybe if you ignore that user, they can't comment on your posts? This is a problem to be solved.
I'd rather visibility to the community as a whole to be a transient thing, dependent upon the quality of your most recent posts and comments, and not upon a legacy. Of course, a user will keep most of the followers they have earned, but if they want to get more of them, they have to keep providing good reason for it.
I am all for enabling a user to filter their own experience, however. So if you are ignoring someone, it would make sense that their comments would drop closer to the bottom in your view of a post. As for general moderation, if and when we get into that, I'd like it to be as dependent upon user input as possible. Ignoring does give a pretty good metric for that, at least to raise a flag that there might be an issue.
I'd ignore them, so their comment would sort to the bottom, and if others ignored them, the same would happen. My guess is that they would run out of steam eventually. If we don't take the good with the bad, then we are left to determine which is which. If it became real problem, I'd probably consider some other approaches, but I don't think civility can be enforced very well.
You're right about civility, it can't be enforced.