Deleting a good start to a review seems like a bad idea
Indeed. But you don't delete the post, you vote to delete it. Just do it. And downvote.
You could also come across one such answer outside the review queues. Often, this results in [correct] NAA flags. Let's be clear: a code dump is not a review, and therefore not an acceptable answer.
However if the post already has a mod notice, I'll dismiss an NAA flag, because short of outright mod-deleting the post (which can then only be un-deleted by a moderator), there's not much else to do for a moderator.
The correct course of action should be to downvote until the score is low enough to vote to delete, and then vote to delete - in an ideal world moderators wouldn't even need to get involved with those, Trusted Users (20K+) should be able to handle it.
Of course, if nobody did before, comment and make sure the author gets a chance to edit and improve their answer before it gets nuked from orbit.