It is part of your responsibility when you're on jury duty to do precisely that.
Is that part of the instruction given at the end of a trial? The judge tells you, "Oh, by the way, if you think that this law they've charged the defendant with violating is really not kosher, according to your interpretation of the Constitution, then by all means, acquit."
If that is the case, there would never have been any 12-0 decisions from a jury who must make a unanimous judgement, those jurors all having an individual interpretation of the law. When the Rev. Dr. Martin Luther King, Jr. found the law of the land to be in violation of his idea of just and right, he broke it and took the punishment. He was justified in his thoughts, but full well knew the consequences of his actions, why it was called civil DISobedience. His sacrifice ended up in getting the law changed by Constitutional method, not by having the law ignored.
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There's a catch 22 though. -- If you tell that to the judge, he throws you off the jury.