Flag Declined and How to avoid it

I flagged an "answer" in this question as not being an answer. It seemed clear to me when I flagged the answer as not an answer, that the OP who didn't have an account was answering a question given in the comments. But my flag was declined because "flags should not be used to indicate technical inaccuracies, or an altogether wrong answer". In cases like this should I flag it with other and include more of an explanation as to what I think is wrong rather than just flagging as not an answer?

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I apologize. That was my mistake. I did not see that it was indeed an answer to one of the comments. Unfortunately, there is no way to undecline a flag. – robjohn Jan 22 '13 at 9:34

Flag again? ${}$ – Qiaochu Yuan Jan 22 '13 at 6:20