Cancelling a call can occur any time a call has not had an officer on the scene, at the start of a call, after the call has been dispatched or the Officer is enroute. The call can be cancelled by clicking the Cancel Call button:
Once an officer has arrived on the scene (or one of the officers on the incident has arrived on the scene), the call is no longer able to be cancelled.
At that point, the officer will need to be cleared from the call normally.