You're conflating two issues, and it might help to differentiate them. Your title and the body of the question aren't necessarily the same thing.
Not all "unanswerable" questions are actually unanswerable on a Q&A site. For example, questions based on misconceptions or thinking about a problem incorrectly are often recognizable, and a legitimate answer is explaining why the question is unanswerable. That can be just as helpful to the OP and other readers, especially if it involves common misconceptions. Similarly, subject matter experts may know that certain questions are not currently answerable for one reason or another. Again, explaining that is a legitimate answer.
Those kinds of explanatory answers would make it an answered question, probably on most sites. Just because it isn't answerable doesn't mean it isn't answerable. :-) So the title doesn't accurately introduce the actual question you're asking, and was a source of confusion for some readers.
The body of the question makes clearer what you're actually asking. What you describe is different, and it focuses on a specific type of post -- someone using an answer post for what is essentially an opinion comment that the question isn't answerable ("opinion" because it includes no supporting discussion or evidence).
What you're really asking about is a kind of "non-answer" in an answer post. And because the question is one that is not answerable, this has additional ramifications. It has the side effect of messing with how the question is treated (no longer "unanswered" because there's an answer post, possibly complicating getting it Roomba'ed, etc.).
Clarification of impact
Just FYI, the actual operational impact may not be as great as you might anticipate. For example, assuming nobody upvotes such an answer, the question would still appear in the "Unanswered Questions" list. But you're right, it should not be treated as "answered", and it does have some effects on the question's handling or potential handling (not to mention that it can be irksome to see that as an answer, and it doesn't help the site's content quality).
So how do you fix it?
You were on the right track with moderator flags, but you need to accurately classify the issue and use the right type of flag. Some insight into the process:
Even with the wrong type of flag, there's a good chance the moderator will recognize the actual issue and take the appropriate action. But their feedback to you will reflect whether the flag was correct. If nothing else, that serves as a teaching tool to improve the quality of flagging; "you flagged it as X, and that is not an example of X".
What can make picking the right flag a little tricky is that NAA and VLQ have specific definitions that don't cover things people often think ought to be included. And whether or not a specific answer meets the definition can be somewhat in the eye of the beholder. The safe flag for the case in this question is a custom flag (needs moderator intervention), in which you can briefly explain why the answer should be a comment or deleted. The moderator can choose either action, the result of which will be that the question is again unanswered.