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If they're on the Internet, someone has a copy of them. They are free now, and you will never have full control of them again. I won't swear to it, but I think when EL James got her book contract...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/15920 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/15920 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
If they're on the Internet, someone has a copy of them. They are free now, and you will never have full control of them again. I won't swear to it, but I think when EL James got her book contract for the _Fifty Shades_ trilogy, she deleted all the posted versions of those stories (which were after all _Twilight_ fanfic). I seem to recall that older versions were available in various online archives (maybe the Wayback Machine), so deleting them from wherever they were hosted didn't make them go away entirely. The upshot is that once you make something public, there's no way to completely prevent people from accessing it, and once they can access it, they can plagiarize it.