To cancel automatic renewal go to "My Domains" page or Billing via your Account's page and disable the auto-renewal. Domains are valid for 1 year from date of purchase. They are automatically set to renew unless you cancel the auto renewal.
Note that the custom domain info will only show to the admin that purchased the custom domain.
We suggest making a note on your calendar as canceling is your responsibility and domain charges are nonrefundable.
For domains purchased directly via support, your domains will not renew automatically.