Pro-rata Cancellation
When the policy is terminated midterm by the insurance company, the earned premium is calculated only for the period coverage was provided. For example: an annual policy with premium of 100,000 is cancelled after 40 days of coverage at the company's election. The earned premium would be calculated as follows: 40/365 days X 100,000=.110 X 100,000= 11,000.

Short-rate Cancellation
When the policy is terminated prior to the expiration date at the policyholder's request. Earned premium charged would be more than the pro-rata earned premium. Generally, the return premium would be approximately 90 percent of the pro-rata return premium. However, the company may also establish its own short-rate schedule.

So you have only one option Short-rate Cancellation.
Source - about.com