On the web:
1. Go to my.roku.com/account and click on 'Manage Your Subscriptions.
2. Find your Showtime subscription and click on 'Cancel'
On your Roku:
1. From your home screen, find the Showtime channel and press '*' on your remote
2. Select 'Manage Subscription'
3. Select 'Cancel Subscription' from pop-up window
Note that if you cancel while still in your free trial period or before your expiration date, you will continue to have access to the service until the trial or subscription period ends.