How do you know that a student cheated? It must be more than a sick feeling in your stomach. Document the events. Include dates and the situation. If you suspect a student, start keeping records so that you will have a case if and when something tangible develops.
Not all situations will be clear-cut, but some will. If your student fails a turnitin.com analysis you will have the evidence you need. Other cases will be more difficult, but if you have been diligent in your record-keeping, your case should hold.