IANAL but I think legally, if you put CC-BY on it, then just write in the comments that you can't use it commercially, that wouldn't stand. CC-BY-NC is available, and the author did not have to upload it as CC-BY in the first place.

To be clear, I still think this site has some problems, and author's wishes should be respected, but I think if the author wants you to not use it commercially, they should put the correct license on their work.