Push to shove, if you posted it first and someone else claimed it as theirs elsewhere? I think you could probably talk to the moderators elsewhere, send them the link to your original post, and ask them to delete it, and possibly bar the plagerizer.
But I think that sort of thing would be pretty rare. There's no money in it, and its too easily punishable if you try.
To worry about someone stealing a story you made up about a character if posted online? Do not take this as I think I am a great writter. I was simply curious.