You can stop a domain you bought for your custom email address from renewing yourself, from your dashboard.
- Open the Custom email page in your dashboard. Sign in with the Gmail account you used to buy the domain.
- Click the ••• button next to New custom email, then click Manage domains.
- Click Manage Domain next to the domain.
- Turn off Autorenew.
Do it before the Renews on date shown on that page. That's when the domain is charged for the next period.
Note: Delete in the menu next to an email address only removes that address. It doesn't cancel the domain or its renewal.