You can stop sharing your vehicle at any time via the Westfield app.
If you are the primary owner of the vehicle: Go to the Parking screen, and for the vehicle's number plate that is shared, select 'Share number plate'.
From there, you can remove the person you no longer wish to share the vehicle with. This will remove the vehicle details from their Westfield app.
If the vehicle has been shared with you: Go to the Parking screen, and for the number plate of the vehicle that is shared, select 'Remove number plate'. This will remove the vehicle details from your Westfield app.