It's a great idea that I like a lot, along with a block feature that mk and b_b were talking of earlier. My bigger issue is the image that is put out. The asshat we're talking about has made himself present in every post recently, so new users first experience is going to be... a vitriolic racist red piller. It would still be great, because as users will go on, they'll see that people have applied the Mutter option to him so much he never gets responses.... except from new users who don't know yet. Imperfect, but it's still a step that would do a lot.
Posts highly-muted people make are hidden from global, comments on others' posts are not though. But then again, false positives in posts on global are only slightly bothersome. False positives on highly-muted people having their comments would be a lot worse (and easily abusable).