Politely ask them to take it down. If that doesn't work, make some noise and ask others to write to them and politely request that they take it down. If they persist, mention that your work is copyrighted and you have relatives that are lawyers. :)
Ask them to take it down, and publish their names far and wide as no-good scoundrels if they don't comply. This is kinda what fannish shaming is *for*. (I am not generally in favor of shaming, but posting someone's work w/o permission is on the Not Okay list.)
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