I see flag as something more along the lines of report on Reddit - I assume it raises the flagged comment to the actual moderation team (dang et al) for review & action against the user if necessary. I don't think it's worth flagging someone and risking their account over a bit of self-promotion, punning, one line jokes, or abrasive comments. I see downvotes as my chance to shape the conversation without jumping in to comment and add to the fire.

An example: a common HN trope is to comment something tangentially related to the linked article, but the comment isn't directly related, isn't an attempt to discuss the linked item, and comes off with a "hustle" vibe (check out my project that is similar, check out the company I created that solves the same problem, I wrote an article about this last year go to my website instead) rather than actual discussion about the article. Sometimes these comments come off as extra slimy, but I'm not going to flag someone for trying to get attention, I'm just going to do what I can to shape the conversation by downvoting in cases I judge as egregious.