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We don't have the manpower or resources to plagiarism test 300-400+ articles sent in daily and @cheetah already does this.

Oh! Such numbers are impossible to handle. You are right.
But having a "black-list" at least gives you an idea of which users to be more careful on.
Perhaps a collaborative effort and some brainstorming will work it out. If I can think of anything, I'll definitely contact you.

There is a global blacklist project underway and we have a local blacklist as well. It adds a lot of systematic issues though.

Sure, as the scale grows larger it gets even more difficult to control things.
I wish you good luck and keep working the way you do! @buildawhale is one of the best services around :)