I may be being a bit harsh, but I've seen a lot of "stupid" answers across StackExchange where the user simply hasn't read the question carefully enough and answers willy-nilly.
Recently, I had this as an answer on one of my questions:
Why is vlc not syncing audio and video correctly?
Like I said in my comment, the answer simply isn't relevant to the question. Is it OK to flag answers like this?
That question is practically a case study in answer flagging; I killed all three answers on it. There was one that offered a solution if you're using Pulseaudio; I would've left it except you commented that you weren't. I edited that into the question, which made the answer inapplicable, so I converted it to a comment (I probably could've just deleted it, but it at least led to an edit). Another asked a couple clarifying questions without any "and here's how you fix it", so I converted it to a comment as well. The third (which you linked to) offered a workaround you already mentioned in the question, so I just deleted it
So in short, yes, you can flag answers that don't actually address the question, or don't offer any solutions. However -- and people get this wrong on other SE sites constantly -- you can't flag answers for just being wrong
Example question: How do I list the files in a directory?
Answer 1: You can list files with the rm command
Answer 2: You can remove files with the rm command
Answer 3: Which distro are you using?
Answer 1 is wrong, but tries to answer the question, so you should downvote (and maybe toss in an "uh, no, that deletes stuff" comment). Answer 2 is answering completely the wrong question, so you should flag it as a non-answer. Answer 3 isn't an answer at all, so you should flag it as a non-answer and we'll convert it to a comment on the question
rm example, answer 1 goes beyond just being wrong, its actively destructive (maybe even malicious), (as Jeff mentions in his answer). That seems like a good reason to flag. I suggest, "you can list files with the cat command" is a better example, as that's not destructive (and plausible, as it means the answerer misunderstood "list"). This question hasn't had much attention (66 total views as I write, a single upvote on the question and a single vote (up) amongst all the answers). You could have flagged at least one of its answers as a non-answer, perhaps two.
That answer you link to (“You can resolve this on a case-by-case basis by using the j and k keys”) is squarely a non-answer (it's clear from your question that this is a general problem and you want a once-and-for-all solution).
Another answer (“When you have high cpu use the video will get behind the audio.”) is borderline between non-answer (→ delete) and unhelpful (→ downvote). It would be a worthwhile suggestion if you'd said that some videos had this issue, but since it's all videos, that shouldn't be the problem.
On a more general note, a guideline between downvoting and flagging as “not an answer”:
No -- a stupid answer should be downvoted and commented on but not necessarily flagged. (And if editing the answer is possible to make it less stupid, do that too!)
I would only flag if the answer is actively dangerous or materially harmful in some way.
