I flagged https://travel.stackexchange.com/a/81153/4188 as "This was posted as an answer, but it does not attempt to answer the question. It should possibly be an edit, a comment, another question, or deleted altogether." because it does not even attempt to answer the question and indeed it should be deleted. What value does this answer offer and how can I react to a disputed flag?
Update: The answer has now been deleted by OP.
It does not even attempt to answer the question.
Yes, it does. Saying "No, it doesn't exist" is a perfectly valid answer.
However, this answer cites no references or reasoning. It doesn't even attempt to explain how it arrived at the conclusion. This is very unfortunate for an answer claiming a negative. One needs to show a lot of research and present very strong arguments in order to credibly be able to claim a negative. It's therefore a poor answer and I have downvoted it. However, NAA should not be used for answers that are simply very low quality. We have the VLQ flag for that.
OP should be encouraged to improve the answer by adding reasoning, research or sources.
What value does this answer offer?
Very little, because it doesn't explain how OP arrived at the conclusion. It's very low quality.
As is, the answer should certainly not be deleted as NAA. It's technically an answer.
If anything, it could possibly be deleted as "very low quality". I would certainly not oppose that. But personally, I'm content just to downvote the answer. OP can consider improving it.
How can I react to a disputed flag?
By doing exactly what you have already done, posting on Meta.
By the way, I recently got a truly useful answer regarding a very similar situation on ELU. It made me rethink my philosophy regarding deleting answers. Sure, it's a different site, but the principles are very similar. I hope you find it useful.