It makes sense to me for someone to down vote the most top-level comment that led to an off-topic thread. But then further down voting comments that are replies

It makes sense to me for someone to down vote the most top-level comment that led to an off-topic thread. But then further down voting comments that are replies within that thread makes little sense, and could be mistaken by some as vindictive down voting (which is likely why the automatic ban you describe exists). Further, if there are enough replies in a given thread such that your down votes on all of them were enough to lead to a ban, it strongly suggests that the thread really was on-topic, that it generated valuable interest and discussion, and that actually it's just a controversial thread (one you may not like perhaps), but clearly not off topic. I do see this a lot on HN. When a thread is controversial in a way that HN group-think disapproves of, then "off topic" is used as an inappropriate excuse to down vote it, try to derail any discussion about it, etc. For example, I think effectively all of the stuff from the subreddit TheRedPill is utter nonsense, and it's often very controversial and I dislike it when some of it appears here at HN. But it's rarely actually off topic when it's brought up, and it generates actual community interest (usually from people who rightfully point out how problematic and baseless it is). I'd rather that controversial topics and tangents, even when they are things I don't like, are permitted when they clearly generate community engagement, than to basically have some implicit HN blacklist of disallowed topics that are facetiously treated as off-topic as a way to simply exclude unapproved opinions.