Let's say you went to physics SE and asked how to build a pair of wings that you could strap to your arms so you could fly.
Someone would probably post an answer saying that it is physiologically impossible for human arms to generate enough force to provide lift for the human body and therefore there is no way to build such a set of wings. That answer would probably get a lot of up votes. But you would not be any nearer to being able to fly.
Should you accept that answer? The convention seems to be that you should accept the answer that is most useful to you. What does useful mean? You are no nearer to your dream of flight, so in that sense it is not useful and you should not accept that or any other answer that does not enable you to build working wings.
You may also continue to believe, that answer notwithstanding, that humans actually can fly. In that case you believe the answer to be incorrect, and you should not accept it.
On the other hand, you could decide that the answer is helpful in an unexpected way in that it enable you to finally give up your dreams of flight and take up SCUBA diving instead. In that case, you should accept it.
If you decide not to accept any answer, should you delete the question? Questions and their answers on SE are not for the person asking alone. People vote on questions and answers to say that they find them useful. If your question and at least one if its answers have a lot of votes, that means other people found them useful. Therefore you should not delete the question. Others may benefit from it even if you don't.
Also, it is possible that someone may come along later and point out that by consuming large quantities of Whizz Bang Protein Powder(TM) you would be able to generate enough lift to fly. Or they might give you the plans for a hang glider. As long as your question remains on the site, there is always the possibility of a better answer being created.
You might have to wait five years for an answer you are willing to accept, but that is fine. The question is still legitimate, even if the consensus answer is "you can't get there from here", and there is always the possibility of someone coming along later and proving that you can.