The answer to your question is in that thread right there.
If you don't feel like clicking the link, here's the gist. You can try to block it into being flat by wetting it and pinning the edges flat against something like a styrofoam board. You can try to go back and add a border to see if that will help to uncurl it. But the only way you can be absolutely certain of having a non-curly scarf is to redo it with a gaerter or seed stitch border.