Oh, No! That novice paddler has just veered in front of you. You're about to hit! Where are the brakes on this thing? You should be able to arrest your kayak's forward motion in four strokes. Here's how.

Rotate maximally and plant the back (non-power face) of the paddle flat on the water near the stern. Push down, hard, with your onside hand. Don't hesitate to commit your weight - you'll get a lot of support. Rotate toward the midline and straighten up as the paddle comes to your hip. Repeat on the other side. The third stroke is a strong reverse stroke (you won't need to lean on the paddle so much), and the fourth is a normal reverse stroke.