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I dunno... All this will do is ban all the creationists that don't cite their sources. While I too dislike plagiarism, this seems primarily reactionary, and an attempt to have a reason ready to ban creationists who come here (which will of course give them the recourse of "I got banned because the evil atheists know I'm right", though I suppose any banning for any offense will give them that excuse).
Give them a warning first, then a ban. Indeed the rule doesn't have to be enforced to the letter, but continual posting of plaigerised material should be a bannable offense. It's then up to admin whether or not to act on it.
Seems a bit silly to enforce such a rule to me. If someone does try to plagiarise, and people notice, then they look like an idiot. We don't want some situation where people get banned for being too lazy to bother with source-citing.
If someone on the thread wants a source, they can ask the poster for it, and if they don't, they lose credibility in their argument. It's not illegal to make a statement without backing it up, just inadvisable.