You can cancel your free trial any time during the trial period, and your access to STARZ will continue until the date that your trial expires.
For example, if you are on a 7-day trial, you can cancel on day 1 and still have access to STARZ until day 7. Then your subscription will end, and you will not get billed further.
You can cancel your free trial through the same method of canceling a paid subscription through your account settings as shown below:
Remember it is your responsibility to cancel before the trial ends to avoid being charged.