Add a speed bump
One thing you can try, even now on the finished scarf:
Pick up stitches all around between the garter stitch border and the stocking stitch area. Then, bind them off immediately. Meg Swansen calls this a "speed bump" and it has worked like a charm for me to prevent rolling of stocking stitch when a narrow garter stitch edge was not effective. It's super easy, you can do it in a contrasting color or match it if you still have yarn. It actually looks great, it's a design element!
"One can never have enough socks."
A. Dumbledore, Book One, Chapter 12