It's simple to remove your ad once you have sold your vehicle:
Sign in to your account and click on the blue ‘Remove ad or Mark as sold’ button.
If you would like to just 'Pause your ad' you can do so by following these instructions.
If you happen to have more than one vehicle for sale, just select the vehicle you want to remove via the 'Select your ad' drop down at the top of the screen.
You can always reactivate your ad if you end up needing to advertise it again at anytime free of charge.