I'm not a lawyer but, I've spent my entire career dealing with contracts both in the board room and in the court room.
Any verbal cancellation is technically enough. The problem is that if it goes to court, it's a "he said, she said" situation. You don't need any special form. Any written documentation from the customer cancelling the contract is sufficient. An email is the easiest. The original agreement is now null and void.
A solid boiler plate agreement should contain cancelation information and the course of action in the event of a disagreement. Examples of info might include which county a legal dispute would be resolved, mediation, consequential damages, etc.
If the customer reschedules, a new agreement will need to be created.