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When I read the title, I was hoping this would give the thread owner more control over who gets to post there. Excluding users who are on ignore by the OP would be a good measure against spam.
Posts of the users who are on ignore won't show up, only an empty post with grey color, so it's just a few clicks to mark them and delete them with a press of the button, you don't have to read them.
I know it will raise the deleted post count in the statistics, but that's not a real problem. If you don't let ignored users post in a selfmod topic it can also be interpreted like their posts were deleted right after they have submitted it, so no much difference...