My personal thought on that is I would. Especially for repeat offenders. Though I can imagine something like that can quickly become a game of cat and mouse. But what I bet those sorts of sites don't have is the resources to compete against a crowd-sourced effort to keep those sorts of sites off HN. Not that people would be looking for offenders while browsing stories, but it's likely with awareness people would take time to verify unfamiliar sources that are submitted to the site. At least it forces owners of sites like that to scramble if they realize they can no longer get the clicks they used to get from HN. Especially the ones who are just spamming content in order to generate clicks.

It appears there have been a lot of submissions for that site in the past, so if they're a repeat offender it would probably be easy to extract that info.

You guys probably have a treasure trove of data where you could probably extract some of the more obvious offenders pretty easily. Cross referencing submissions by specific individuals for specific domains, etc. Though I imagine there would be plenty of false positives...

Also, just a thought, I would suggest that the warning message to submitter (if they tried to submit new URLs from banned domains) would be that the site has a history of plagiarism. That way they would probably do additional work to find the original (or more original) link than the one on that site. Also that way in case a site owner wanted to object they would know the reasoning behind the ban.

I know HN has been around a long time so I am probably re-hashing some of the things you've already got in place, but figured I'd put some thoughts out there in case you guys didn't already have something in place.

Would you suggest reporting these sorts of things via email in general?