Flagging is for
- a post (or comment) that needs immediate attention by a moderator: for when the post (or comment) should not be on the site longer than absolutely necessary. E.g., an insulting post, an advertisement. Also, it's for
- something in no way reworkable into a possibly valid answer. E.g., gibberish, something totally off-topic.
IMO, an answer that doesn't address the question doesn't necessarily fit that bill; and, in particular, the three you link to don't fit it. However,
- if you have enough rep, you can comment on the answer, advising how it might be improved;
- if you have enough rep, you can downvote the answer; and
- if you enough rep and the answer has a net downvote, then you can vote to delete it. Enough delete votes, and the answer's deleted, even if a moderator doesn't get involved.
Note, though, that this is my own opinion. It's not an official response; perhaps one of the SE employees can provide that.