We do restrict comments by new accounts in some ways, but the cost of restricting them severely would be much worse than the benefits. For example, consider Alan Kay showing up the other day to comment on Xerox PARC history: https://news.ycombinator.com/item?id=8618736. That's worth putting up with a lot of trolls!

Our current approach is to ask users to flag comments that should not be on Hacker News, and to judge by the GP it seems to be working.