Our five month old, 3.2 lb, Sophie does this too. We've tried to stop her to no avail. The vet said the only way she would learn is if we never let her on the couch, even to snuggle on our laps. She won't know the difference between the back of the couch and the seat of the couch. So if you don't want her on there, don't let her on it at all.
We've given up trying to stop her, hopefully she'll be okay
However, I've noticed as she's getting older she likes to play on the floor more so maybe she'll grow out of it, I hope!!