I am creating a wall like this:

// Wall
        var bottomWall = new cp.SegmentShape(this.space.staticBody,
            cp.v(0, this.bottomWallHeight),
            cp.v(MAX_INT, this.bottomWallHeight),
            wallWidth);
        bottomWall.setElasticity(0);
        bottomWall.setFriction(0);

        this.space.addStaticShape(bottomWall);

And making a physicsSprite shape like this:

// Create and add the physics shape
        var carShape = new cp.BoxShape(this.carBody, carSize.width * scaleCarSize, carSize.height * scaleCarSize);
        carShape.setElasticity(0);
        carShape.setFriction(0);
        this.space.addShape(carShape);

And then when a button is pressed I apply this force to the phyics shape:

// Apply down impulse on player
    downImpulse: function () {
        this.carBody.applyImpulse(cp.v(0, -500), cp.v(0, 0));
    },

Then when the carBody hits the wall segment, it bounces back and creates a constant force the other way. How can I make it so that the car just stops from moving any lower and stays against the bottomWall?