I would say in this specific case, the flag could have been handled better. If the OP stated that they alredy did something, then the answer is superfluous and is noise and should be removed.
That said, it would be difficult to say all the things that you shouldn't flag for (as we can keep adding more and more specific cases to the list). However, I'd make these two recommendations based on the somewhat more specific decline reasons that moderators have for flags:
Flags should not be used to indicate technical inaccuracies, or an altogether wrong answer
Moderators are not arbiters of the veracity of legitimate answers to a question. We should be the ones to say "OK, your answer isn't applicable at all to the problem" (think someone responding with an ASP.NET answer to a PHP question) but we don't say which answer is better than any other (that's for moderators and users alike to decide with the use of up/downvotes). Just because an answer is wrong doesn't mean it's not an answer. It's an answer, it's just wrong and having that around can be just as important as having the right answer (as a signpost for others for what not to do).
Flags should only be used to make moderators aware of content that requires their intervention
Admittedly, this is kind of vague, but it's most often a way of us saying "hey, you can handle this stuff yourself, or it's not a concern". For example, sometimes we get flags regarding editing of posts, but people don't realize they can make suggested edits (sometimes we'll use a custom message to indicate as much, but this response is applicable here as well).
That said, flagging can sometimes be more of an art than a science. I'd recommend the following resources if you want to see the history of how various flag situations are handled: