You're right, except there are even more accounts. I killed them all. I don't know what made the guy go on a trolling rampage after all this time, but it's stopped now.
You should just send me an email when you suspect things like this; I can verify it a lot faster.
Wouldn't it be a good idea to silently hide the posts from everyone except the troll. Now he (or she) has been public outed he'll probably try again with a new set of accounts.
The thing is, Hacker News has too small an audience to make gaming very worthwhile. I think that clamping down once is good enough to send the message across. I'd hope so, anyway, since it takes a lot of effort to get to the point where you can vote things up and down.
Do you have an automated system to verify an account is being used for this sort of astroturfing? If so I'm sure many of the folks here would be interested in it. I think a service that helped forums identify astroturfers could even be the basis for a YC company.
I would imagine simply grouping accounts by their ip address, creation time and set of articles they comment on should easily catch the unsophisticated guys.
You should just send me an email when you suspect things like this; I can verify it a lot faster.