To focus on your abs when doing stability ball crunches, the ball shouldn't move under you at all. Instead, keep your knees bent at roughly a 90-degree angle throughout, and flex and extend your spine over the ball in a controlled manner. If the ball is rolling back and forth, it's primarily your knees (bending and extending) that are driving the motion, not your abs.
The Right Way
Lie on a stability ball with the ball in the arch of your lower back and hold a weight directly above your chest, arms out.
Do a crunch, reaching towards the sky. Pause for one or two seconds at the top of each rep.
Don't sit all the way up (with your torso perpendicular to the floor). That takes the tension away from the abs.
Under control, reverse the motion and allow your abs to stretch over the curve of the stability ball. Don't allow your neck to hyperextend in the bottom. Try to keep it in a fairly neutral position.