US copyright law doesn't allow for protection of something like that. A dingbat, yes, but if it's very plainly recognizable as an X then the exact shape and output of that typeface isn't protectable. You can even print out a font, scan it, and create a new copycat font from it. The only thing you can't do is reproduce the actual typeface file itself, which is fundamentally a single copyrighted piece of software. Some other countries allow more protection on the shapes of individual letters, but I don't think you'd ever win a case anywhere on such a simple geometric shape as this X.