Except you actually can, and many states have

You literally can, just like any number of other valid bases for objections to arguments put forward. If the judge rules it to be such a defense, it would be struck from the record and the jury instructed to disregard it, and if the lawyer keeps on it, they would be held in contempt of court. Furthermore, if it is plainly a case of such a defense and the judge lets it fly, the prosecution can claim mistrial.

Perhaps there are other ways of banning it, but that is the obvious one in the American framework.