Trolling and spam are both self-perpetuating problems. Users are ruder on sites where everyone else is rude, and spammers are more likely to submit links to sites they get traffic from. So you can prevent both problems by never letting them get a foothold.
Deletion doesn't have to be manual, especially in the case of spam. Spammers smart enough to measure the traffic they get from HN quickly give up. And the dumb ones obligingly continue to post from banned accounts and IP addresses. So currently 80-90% of spam is killed by software rather than humans.
Flagging turns out to be a feature that saves a lot of work. So does rate-limiting submissions from newly created accounts (and, obviously, the IP addresses they use).
One general approach I've found very useful is not to protect against a certain type of abuse till it arises. Aside from obvious things like not letting people vote more than once, you don't need much protection when you first launch.