I like the idea, but only to a limited extent. Maybe a way for a "thread moderator" to mark a comment "off topic", then allow people to choose how/if they want to view off-topic replies. You could have a "never show", "always show" and "show placeholder" that has a clickable to show the OT comment when clicked?

That prevents OPs from just deleting content they dislike on their thread, but allows them to "filter" things for people who don't want to read all the OT garbage.