Stop an object in motion with events

I'm going through the 'Starting out with Alice' textbook and reach an exercise where I have make the a car go around a circular track when you click a button, then stop in its place when you click a different button. However, I can't figure out to make the car stop

Anyone know a way to do this? I've read the Alice book through and through but can't find anything on it. I'm thinking I missed soething.

Thank you