Flagging as "not an answer" is always appropriate, it will ensure that a moderator deals with it at some point if the community doesn't. If the community manages to delete the answer themselves, the flag is automatically dismissed as valid and no moderator called for something that is already dealt with, so there is no drawback to always flagging here.
I personally don't downvote non-answers from 1 rep users if they are just misunderstanding how the Q&A system works. I downvote for crappy content, rants or sometimes if it is from users who should really know better. You'll get the reputation back after a recalc if the post is deleted, so that is no reason to hesitate. If you want to vote to delete you might need to downvote it to get the score to -1.
One step you didn't mention is commenting, for new users I find it essential to explain what exactly they did wrong. The problem as a normal user is that they won't be notified about the comment if the answer is deleted quickly. There is an exception for moderator comments, those will always cause a notification when posted shortly before deleting.
If I had 20k I would also vote to delete. If there are enough 20k users on the site, mods don't even need to be involved for this. Though if I would vote to delete, I would likely comment to explain what the user did wrong, if he's new.
So with a few caveats I would generally say that you should do "all of the above".