Contact the Transperth Infoline on 13 62 13. They will be able to place a stop to anyone using your SmartRider by placing that card on a HotList. In 24 hours the card will be disabled. To apply for a replacement card, you will need to apply the same way you did the first time i.e. through this web site or with a manual form.
SmartRider online order service:
https://smartrider.monitorwa.com.au/
Student concession SmartRider application form:
https://www.transperth.wa.gov.au/Portals/0/Asset/Documents/SmartRider/Student_SmartRider_Form.pdf
Please follow the "How to apply" section on the 2nd page to complete the form and submit the order.