You say that flagging is only for spam. It isn't. Flagging is for things that don't belong on HN. Some things are obviously not suitable for HN. Somethings are obviously suitable for HN. The tricky area is things that start off being suitable for HN, but then quickly collapse into mindless bickering. There are some words that act as triggers to poor discussion. I proudly flag threads full of bickering. You should too…
If you think something is spam or offtopic, flag it by going to its page and clicking on the "flag" link.