I've flagged certain answers as for moderator attention, noting that they should be comments. However, do moderators have any power to do anything in these situations, or should I instead not flag and just comment on the answer.
We can't transform it from an answer to a comment (by the same author), so in most cases I just leave a comment suggesting that the author make it a comment themselves. If it's a truly content-free response, or if the author's account has been abandoned, I might just delete it. The newer the answer or user leaving the answer, the less likely I am to delete.
They can delete them. If the content is especially valuable, they could potentially add it to a comment, but there's no automated support for this.
I would simply flag the answer as spam, which gives the user an immediate (small) hit to their reputation and has the potential of giving them a bigger hit if 5 other people agree with you that this is spam.